Hi there, welcome back to the Hobby! I play a lot of Kill Team, mostly using my Orks or my Marines. You offered 3 great choices for how you want to expand your Kill Team.
But 1st, what I called Step 0, I just want to check if you have the official Kill Team supplement (
http://www.blacklibrary.com/games-workshop-digital-editions/Kill-Team.html).
I know you said you have the rule book, not sure if you meant just the new 7th ed book or that one as well. I'll be commenting with that supplement in mind:
Deffkoptas - they're great for their speed, ability to draw First Blood, and their anti-vehicle capabilities (but you already have Tankbustas).
Ork Bikers - they're also great, like a lower points cost alternative to Deffkoptas with less anti-vehicle (but you already have Tankbustas).
Ork Boyz - they're great to overrun the enemy with. Shootas were the conventional choice in 6th, but I personally slightly prefer Choppas as they're cheaper now than Shootas (due to the new codex) and melee seems to play a greater role in Kill Team (for multiple reasons). Their low points cost also allows you to horde up, and in Kill Team quantity is a quality of its own (Kill Team is very tactical and just having more pieces allows you more flexibility and a safety margin for error/bad luck). The only other armies that can effectively do hordes in Kill Team is the Imperial Guard (Tyranids actually have a problem due to Synapse Creatures and Inquisition, while Acolytes are actually really good for their cheap points cost, can only take up to 12 models due to
RAW).
Typically, in Kill Team, the Ork's vulnerabilities are enemy vehicles (but you already have Tankbustas) and the Break Test special rule. When your Kill Team has suffered 50% casualties (which almost always happens to both sides), your models start taking Leadership Tests. Orks are
Ld 7 and you can count on, in the later turns (usually by Turn 4) about 40% of your survivors will flee every Turn. Thus, of the 3 options you presented, a mass of Ork Boyz are the best bet. Not only do having more models on the table delay the Break Tests, but even after you have to start taking them, they average out its effects.
Thus, I recommend Boyz. Shootas and Choppas are both good, so don't worry about my personal preference above, instead choose what you will use when you get dead serious. If you do choose Boyz, there is a trick I use I call "the Blood Axe Leader". The Kill Team rules require you designate 1 character model as a Leader, or just any model with the highest
Ld. A Leader is not the hero; he is the easy Victory Point kill for your opponent. For most armies, the Leader is a more expensive figure with the best wargear options (i.e. Chaos Champion, Space Marine Sergeant, etc.), so they don't want to hide them away as it's a waste. For Orks, you can choose a cheap Choppa Boy and just hide him in the corner and just deny that easy Victory Point. If that Boy leaves due to a Break Test, he does NOT give away a Victory Point. I'm both proud and ashamed to admit I've won many close games by having my leader run away like a pansy...at least it's very Blood Axe.
