Ever ridden the 600 bikes?
I was just started riding last season and everyone pushed me away from 600's, and I'm damn glad they did.
I ended up getting an 07sv650s. The V-Twin engine allows for lots of torque down low but is A LOT more forgiving. There isn't much room to correct a mistake on a 600, but this all comes down to the rider...
You don't need anything other than cash money to get a bike though to answer your first question. Most people won't let you test ride one either unless you have the cash in front of them.
Just keep in mind that you'll most likely need at least another 600 dollars to get come gear.
I'm a bargain hunter and got everything 50% off or more and it was all Brand New.
Helmet ~300
Jacket ~200
Boots/ shoes ~100
Pants ~100
Gloves ~60
Keep in mind all that gear is on the very low end of the spectrum for good/ decent leathers.
Most will say that street is way different than dirt too.