Okies, responding to your questions:
1. Shoota boyz are assault 2, 18" range. Its in the codex, and nevermind whatever else you read somewhere. The firewarrior used to be the king of "most efficient model" but the shoota boy has taken over. Six points for a T4 model, STR4 on the charge with an assault 2 weapon. For a foot-slogging unit, these are the way to go.
2. Ghazghkull works in a foot-slogging army too. Ghazghkull has slow and purposeful, which says to treat the model as if its moving through difficult terrain (
2D6). However, if you read the rules for
ICs, all
ICs also have skilled rider and move through cover, which means that you get
3D6 instead of
2D6. Your units move at the speed of the slowest model, so if you stick him in with a unit of foot-slogging boyz, they move at the speed of
3D6. That's not very slow.
3. Every Ork army list needs some kind of "wtfpwnage" unit. Something that can stand up to monstrous creatures, dreadnoughts, terminator squads....Ghazghkull on the Waaaugh! fills that role nicely. A squad of burna boyz does too, but they're not a good unit for a foot-slogging army.
4. What every foot slogging unit needs is a squad or two of Stormboyz. Read their rules; they go zoom zoom zoom REALLY fast. If you can afford points and want to try something fun/reckless try Zagstruck. Deepstrike and assault on the turn you deep strike (if you're in assault range). Its risky though, but you're Ork.
5. Gretchin benefit from the leadership of their runt-herders. Don't take two units....honestly, the only purpose of grots to to screen your other guys. Take one to absorb bullets. Have them go to ground and get 3+ cover saves if you can so you can
lol while your enemy wastes fire on them.
6. Your biker idea - don't do it. Biker boyz are horribly expensive and inefficient. Biker nobs are the same, for a plethora of reasons I've detailed in other posts you can find here on the first page of the forum in other ork army lists. If you want to take nobs, stick them in a trukk or a battlewagon.
7. Deffdreads count as troop choices if you take a Big mek, but they're still vehicles; vehicles can contest an objective, but cannot hold it.
8. Your battlewagon idea is messed up. Battlewagons are either meant to be stationary shooting platforms or fast-moving assault platforms to deliver assault troops into close combat as fast as possible. If you're going to use them as platforms for Lootas, then there's a few things to take into consideration. Deffrolla is wasted because you're not moving or ramming stuff; armor plates are pointless because neither a 1 or a 2 are meaningful to you while stationary, nor red paint (stationary shooting platform). There's really two configuration options to base your choices on.
A.) Assault Battlewagon: Red Paint, Reinforced Ram(Or deff rolla if your store says they work on vehicles), and a couple of big shootas (add a kannon if you like) - the purpose of the guns is just to absorb weapon destroyed results to keep you from being immobilized and unable to deliver your payload.
B.) Shooting Platform: A killakannon doesn't make you non-open topped, that's just an 'Ard Case. You want to be open-topped so that you don't have to worry about firing points so everyone can fire out. Also, big shootas are wasted because you have LOTS of dakka; if you're using Lootas, that's freakin' 45 guys with D3 shots per turn at STR7 AP4. If you want to add something useful, add rokkit launchers and either kannons or Zzap guns so you have some potential
MEQ killers or vehicle killing. Killa-kannons are *really* personal preference. They're expensive, cost as much as a demolisher cannon for less deadliness, and at BS2, tend to scatter more than usual. However, for a pure shooty list, try it on for size like this: Battlewagon, Killa-kannon, 4 Rokkit Launchers, Kannon.
Take three battlewagons, deploy them in a trident formation like this: \|/ so that front armor is facing across the field. Put 15 Lootas in each battlewagon, a Mek with a
KFF in the center battlewagon. Give him a few grot oilers; your 6" bubble of
KFF extends to the other battlewagons, and if you have the backs of the battlewagons touching, the Mek with the
KFF can attempt repairs on the other guys. The effective control radius of a model extends to the hull of they vehicle they are in (for example, a squad of burnas in a battlewagon can put the flame template at any point on the battlewagon). IE, the Mek in the middle can attempt repair on any battlewagon touching the hull.
9. Deff Dreads are a common flank-protecting unit here. They count as troop choices (that can contest but not hold). Close combat weapon and a flamer - no need to fear genestealers outflanking or anything crazy, and those STR10
CC claws can lay the smack down on anything.
10. That leaves your shoota boyz. Exposed units, but you need to have troop choices to take objectives. Basically, I have these guys advance from cover to cover, puking out massive amounts of firepower if anything gets close enough to be shot at, but I don't dump them into assault unless I'm absolutely sure I can win, and do so without compromising my strategy or army integrity.
That just leaves your shooting. You have 45 Lootas with a 48" range with D3 shots that can pour out up to 135 STR7 shots per turn. You're weak on anti-tank, so you can take some out-flanking deff-koptas with twin-linked rokkits to snipe at rear armor. You've got 12 rokkits across your battlewagons that can toast any
MEQ and double-toughness most of
40k's offerings, and a unit of gretchin surrounding you will prevent someone from outflanking or deep striking and getting free hits on your rear armor for you not having moved. Not to mention at least 60 boyz able to crank out 120 shots per turn....there's a lot of dakka.
I've tried taking a Big Mek with a Shokk attack gun in the mix; that large blast template is also randomly good and bad.
YMMV, so try it on for size and see how you like it.
My advice - take it all like I've described and play a friendly game of annihilation with someone; start simple. They've got to cross the board to you or have a shooting fight, and you're decced out to win that fight.

Get comfortable with having a static firebase and how to run it, then try a game of sieze ground / capture and control. Ease your way into a new play style.

As for your battlewagons presenting front armor across the board, remember that if someone shoots at your vehicle and are in a facing arc they can't see, you get a 3+ cover save. Melta-guns are the most threatening, but anything even trying to get close enough to do that will get a withering fusillade of fire. As for the most common tank-killing weapon in the game (the Lascannon) lets talk about those statistics.
Lascannon takes a shot at you, ballistic skill 4, needs a 3+ to hit: 67% chance to hit
Against AV14, needs a 5+ to glance, 6+ to penetrate, or a respective 33% and 16% chance.
You have a 50% chance of ignoring the result with a cover save.
With a glancing hit, its -2 on the damage table, +1 for being open-topped, so minus 1. You don't care about stunned, shaken, weapon destroyed or immobilized, which means they need to roll a 6 to destroy the vehicle. Anything less, you can probably repair with youg big mek.

That's a 16% chance to do anything meaningful.
With a penetrating hit, its +1 on the damage table, meaning a 4,5,6 can ruin your day, or 50%.
If you multiply all those together, you get this: A lascannon shot has less than a 3% chance of doing anything meaningful to you. I think that's all my typing for the night.