if you buy a new bike chances are they will be disc compatible frames (even lower end bikes) as well has have disc compatible forks. this may be the way to go if you plan to heavily mod the bike in the future.

another route would be to buy used. you can pick up a very decent hardtail used for around $300, but then you might not have a disc compatible frame/fork and this may be troublesome in the long run. but this way you get the most bike for the money in the short term at least.

depending on where you live, www.craigslist.org classifieds sections may be of help.