I bought my Dreamcast on ebay for about $60 shipped. Didn't need a ton of games with it, so that saves money.
I remember when I was looking that they said ALL Dreamcasts could play backup-games (ie. burned). I don't know 100% if that is true, but in reading around it does appear you're okay. If you get an earlier model, you'll be assured of no problems... as only later models had copy protection (if at all).
ALSO, check Craigslist. A number of people on Craigslist will tell you if it can play backup games.