I'm in the market for a new amp to start gigging with too. So far I'd spent most of my time looking at half stacks because Bass combo's tend to be pretty low powered for the most part, mostly just practice amps. As for whether 100 watts is enough it all depends on your band. If you have a guitarist or 2 who play with 100 watt stack that are near to cranked then no 100 watts won't do. It takes alot more power to push the air in a bass amp, that's why you see ludicrously large wattages on alot of bass heads (ex: Bassman 1200w). Speaker configurations depend on you alot too. If you play a 5 string bass you'll likely want a 15" but a 15" doesn't have much definition on high notes. Ideally you'd want a 4x10 and a 1x15/1x18 or something like that to get the best range but if you play 4 string then likely 2x10 or my favorite a couple 12's should work fine. It all kind of depends on what you're looking for.
For $500 you might want to try playing a bunch of amps at your local music shop(s) and then trying to find one on ebay or at a pawn shop. If you can you'll probably get it a fair bit cheaper and a few little scuffs on the tolex don't make any difference to the sound. The Bassman 400 is nice, and it has speaker outs on the back so you can set it on top of a cab for some extra volume if you need to. Ampeg makes really nice heads, and some amazing cab's but I don't know what their combo's are like. SWR makes some combo's with a decent power output for a pretty good price, haven't played one much though so I can't really comment on the sound either way.
Finally don't get too hooked on tubes, they don't make half as much difference with bass as they do with guitar and they make the amp a hell of alot more expensive.
Hope that helps.
Disclaimer: The Preceding message was posted by a bassist, as such take into consideration that his rig has more power than the rest of the band combined.