$3500 will buy you a lot of used bike.
for beginners, the ninja 250 really is a great place to start. i wouldn't say you'll get tired of it, but you may want more power a couple months in.
the 500cc class is an ideal compromise for people who want a 'starter' bike that still provides enough power for long highway rides, etc. (I've been riding my EX500 for 4+ years now).
a 600cc sportbike is not a good beginner bike. 600cc cruisers would be fine though. Just far different power delivery.
the 650 twin class is about as big of a sportbike as you'd want to start off on. If you can find a good used Ninja 650r, or SV 650, that bike would be ok to start on (not ideal, but ok), and still have enough power that you could ride it forever without wanting more.
SV 650 and Ninja 650R are the best choices if you want to buy one bike and keep it forever.
otherwise, look into used ninja 500's, GS 500's, or even the Buell Blast (by far the slowest of the 3).
you should be able to find any of those three for under $2,000 and make your mistakes on a CHEAP bike, then ride it for a year, and sell it for the same $2k you paid for it. THEN go out and buy a 650 or a 600.
I've never heard any rider say, "i wish i'd started with a bigger bike." there are plenty who regret starting on a powerful bike though. just watch youtube, and look at all teh 'learning to ride' videos. the peole on 600's crash a lot more often than the 250 riders. it's not coincidence.