Way to talk in circles
Yes heavy mech is a popular Ork build.
Id say the 2 most popular builds are BW spam and Kan walls. BW spam is as many battlewagons you can fit in a build and rushing everything to the enemy, while hoping the BW armor can hold getting you there. Kan walls are spreading a couple units of kans in front of your army, and using a
KFF to grant them 4+ cover saves, they are REALLY tough to pop with shooting, AND they grant a 4+ cover to the rest of the army behind them, as the enemy has to shoot through them to get to your army.
Kans are more survivable and are a foot slog/horde army. BW spam hits hard and fast. You either kick ass, or get popped early and get your ass kicked.
Onto your first question
Kans and dreads work great while covered by a
KFF (like I said above with the kan wall) dreads usually are left behind simply because you get 1 unit for 1 heavy support choice. While kans you get 3 for 1 heavy choice, and can therefore cover more of your army.
If your not wanting to be a tourny winner, you can mix anything in the Ork dex and have fun. If you want to step it up and get some wins, you dont really want to do that so much. When I take dreads, its when they are troops, because Ill take them with a BigMek. The possibilities of all those kans/dreads on the field can be good, I mean they just munch vehicles of all sizes, can murder
MCs and really smash troops.
Id say if you wanted to mix some buggies in there you have 2 ways to play them. Keep them behind the kans/dreads for protection, or go in front of the kans/dreads to protect the kans/dreads. Buggies are great for the ladder part. They are 35pts for a rokkit buggy for one, so you can afford to have them take a hit. I personally love rokkit buggies because they are cheap, can be strung out in squadrons and grant cover saves to the units behind them, as well as shooting
TL rokkits that can easily hurt
MC/tough guys and bust transports. So if the buggies were to cover your kans/dreads they would have a higher survivability, that way if the buggies get smashed, then the kans/dreads break out and can then hammer on the lines.