I want to know what I should buy to complete it and what would be better left out completely. Money is very much an issue so is there any way I would be able to finish a decent sized playable army for < $100?
It really depends on what you'd like your "theme" to be. Orks are a heavily thematic army, you'll want to pick one and stick with it to keep your spending and additional modeling to a minimum.
Green Tide
The obvious choice based on what you have right now would be to start an infantry army, most times referred to as a "Green Tide" army. You could very quickly and easily start it up with what you have and a few visits to Ebay to pick up additional
AoBR Ork Boyz. (Seem to be going for about $1 a model right now.)
Any games lower than 1000 points means that you'll be fairly competitive (for Orks).
If you don't want to go infantry, then your starting to slip past the "Over $100" spending limit you have asked about. HOWEVER, there are a couple of easy options that are still in your price range.
Mini Kan Wall
You could buy a box or two of Killa Kans (3 to a box) to supplement your Ebayed Boyz and give them cover and additional Rokkits.
Additionally you could scratch build a Big Mek (
HQ unit) out of a Nob to get yourself a
KFF and run a small Walker army.
All under the $100 limit.
Cult of Speed
You could buy two more Trukks, and run a mini speed freek Army, but be aware, small units of Orks are easily wiped out and flee easily as well. Trukks are also easy to destroy with low armor. Still, I've run this kind of list and enjoy it a lot, even though it's tough to win with.
Battle Wagons
I don't think you really have the cash to do this right. Only one wagon will draw all the fire your opponent has, and leave you frustrated that it never makes it to the fight.
All this is stuff you can find anywhere on the web, so I'm just repeating for fun.
also, is it completely necescery to have both bikers and deffkoptas?
You can easily play Orks forever and never touch a bike or Kopta. However, both are really fun and dpending on how you build it, really effective. That said, I don't think your budget is right for bikers just yet. You'll want enough of them to do it right, and unless you can find a great deal, $100 might not be enough to flesh out your army properly.
So, based on what you have, and what you can spend, I'd build one of the following lists.
Ork Horde
Using only what you have with your spending limits.
HQ
Warboss
Troops
20 boyz with 2 big shootas + Nob
20 boyz with 2 big shootas + Nob (Ebay!)
20 boyz with 2 big shootas + Nob (Ebay!)
20 boyz with 2 big shootas + Nob (Ebay!)
20 Gretchen + 2 runtherd (Used to screen advancing boyz) /w purchase of 10 more Grots.
Easily under your limit.
Or
HQ
Big Mek (Scratch built with Nob)
Troops
20 boyz with 2 big shootas + Nob
20 boyz with 2 big shootas + Nob (Ebay!)
10 Gretchen + 1 runtherd (Used to hold objectives)
Heavy Support
6 Killa Kans
Also under your limit!