[Noggin]

Personally, I'd get all I could. I'm using 2 starter kits, 2 hardware kits, extra metal from vexlabs, about 14 motors and 12 servos, 2 wheel kits, 1 encoder kit, 1 ultrasound kit (wish I had a few more) and several sprocket and gear sets. I have one bot that stays in working order, and use the rest of the parts to prototype other bots. I typically have 2-3 bots on the workbench at any given time, and the extra parts/motors make it easy to do so.

Of course, if your only looking for 1 good bot, then I'd say at least 6 motors on hand and 4 servos. You can build most any armature with a combination of those, and have spares just in case (I've toasted a servo already). I'd also get a few ultrasounds if you can find them, but the encoders are kinda useless because they don't record direction and they are bulky. You can code around the direction issue somewhat, but it still isn't optimal. The extra hardware and wheels are pretty handy too.

Hope this helps.