Late to the party as usual but here's my 2 cents.
Consider getting a used Honda RS125. Tons of fun, easy on tires and about a third of the maintenance costs of a TZ or Honda RS250. The GP bikes are much easier to work on than an Aprilia but the Aprilia's are pretty easy too.
ISM gave you the recommended intervals for racers but many people have gone well beyond those limits for trackday-only Aprilias and been fine. But, like he said a seizure could cost you alot more in the long run.
Not knowing anything about your size, mechanical ability or budget, I would recommend getting a Honda RS125 or a Yamaha TZ250. A 125 will be faster than the Aprilia and the TZ250 is way, way faster than the Aprilia.
Having said that, the best way I can sum it up: the Aprilia provides about 80% of the fun of a GP bike and can be relatively turnkey, 2 stroke fun. They seem to hold their value fairly well.
Parts for the Aprilia are not as easy to get as for the GP bikes but most things are pretty available except Aprilia cranks come and go on the market so you might need to plan ahead if you're gonna replace one and order it early.
Expect to pay $3500-$5000 for an Aprilia depending on condition and spares.
Apriliaforum.com as has already been said and for GP bikes:
www.tz250racing.com
www.usgpru.net
www.fatbaq.com
www.wera.com - they have a 2 stroke forum
Recently rebuilt from the ground-up TZ250:
New addition: 125 next to a GSXR750:
