it all depends if u wanna put some work into it or have it ready to go as is.
ready to go will cost you at least 1000 more than something you can customize to your likings.
if you wanna stay out of trouble and account for breaking stuff on it sooner or later (100% going to happen), keep it stock.
Do you wanna have to chase for aftermarket replacement or just go with sand on the beach OEM used parts you can snag off ebay
slip on exhaust is nice to look at, i got one on mine.
expensive upgrade is a no name frame slider where it probably will be cheaper to buy a whole new set of no names instead of a replacement part for a Yoshi or Woodcraft.
milage... don't buy anything above 12k miles or you need to consider more serious maintenance work.
anything under 8k will most likely still have the original Chain and sprockets on ($200 to replace with).
Watch for the age and conditions of the tires. Production date stamp shouldn't be older than 4yrs (3409 = 34th calendar week of 2009) and wear bars in the tire center should have enough left on top and not close to flush with wear bar.
if your 07/08 bike has less than 4k milage you're looking at new battery and most likely clogged/dirty fuel injectors and rust inside the tank from bad storage.
Chinese fairings are a bad sign, so are scraped on bar ends and footpegs or even worse, tank, tail, mirrors and upper fairing side.
so your perfect bike should have 10k miles, fairly new chain and sprockets (4k old), fairly new tires (1k miles), OEM rearsets, original paint (watch for the kawasaki or ninja sticker on the tank with clearcoat on top), a nice slip on exhaust, cleaned up tail (if oem, who cares, just looks) and most importantly frequent oil changes done, every 3k miles. Doesn't even need to be on full synth oil.
little hint for when starting out with that bike... let some air into the front brake system so you have a spongy lever that won't hit the handlebar if adjusted out on the mid setting.
those brakes can stop on a dime if applied correctly (firm but progressively) and no grocery getter cage could possibly brake in time to not rear end you.
don't forget to put at least $500 aside for decent gear. if you skimp on kevlar jeans, so be it. but all that's separating you from 40grit sandpaper and the bike is your 0.1in thick skin plus another 0.05in of t'shirt or a thick sweater that won't protect you from smashed elbow joints, painful weeks in the hospital, light duty work and drastically cut paycheques for the next 4 months besides the medical bills.