You should get something that fits the folowing criteria:
- dirt-oriented suspension
- low maintanace
- road worthy
- smooth power delivery
I'd go for the xr400 or the dr 350 for starters. They're cheap, air cooled and fun. You'll get bored of them in a year or two and find out what you really like: adventure riding or single track.
If you find yourself thirsty for power and fast fire roads...500cc+, otherwise, 250. Hope to have been of use.
p.s: I have a friend riding in the romanian national enduro championship; she started up her competitional life with a wr-f 250 and she loved it(she also had a SUMO setup for it); her current bike is a 250 KTM exc 2 stroke.