For superhero figures, look up batches of Heroclix on eBay (don't pay the exorbitant prices for singles). Should get you a team or two if you want to mix and match, plus some villains to face them. The Heroclix figures can be repainted and the plastic is OK for conversions.
That sounds like a good idea. I've seen some good heroclix conversions around and would like to work on doing some myself and I'll no doubt need a fair few cheap figs to practise on because my conversion success rate isn't too good!