craigslist.com, select your area... you want a USED bike... no matter what unless you have thousands to just throw away, which you don't since you said you're saving...
any used bike, just make sure they can show you maintence records (if they can't it's becuause they didn't keep it up to date.... anyone at all who cares about their bike would have these records, if they didn't care about the bike, they most likely ran it into the ground, remember, maitence is always cheaper then repairs, there's literally no reason not to)
if you get a popular bike, like a ninja 250, when you're ready (and you need to be ready before going bigger) you can sell it for nearly every penny you bought it for.... again, no excuse not to, other then being hardheaded....
you buy a new bike, the second you ride it, the price drops significantly, you lose ALOT of the value.... [generally] buy a 600cc and wreck it, you just lost everything unless you were lucky enough to walk away with just road rash.