1.5, 2, 2.5, 3mm is all you need. I would suggest a set with a lifetime warranty. That does not mean a hobby set which has rediculous markup. Try getting like a Sears/Craftsman set or similar if they are warranted.

I have a cheapy set from Home Depot (no warranty though) that is cool, because it comes with a handle you can attach to the wrenches to give you tons of leverage. Note that I only use it for loosening overly tightened screws. With these cars, you don't need to use Hurculean strength/G.I. Joe Kung Fu grip to put in bolts. Just buy some blue compound thread locker instead, and just barely snug the bolts.

The secret to long allen wrench life is simple. When a screw is clogged full of dirt, insert the next smallest wrench in there to scrape out all of the debris before trying to loosen the screw. If you do that, you will probably never need to test the warranty on your wrenches.