Need your guys input??

So I'm looking to buy and sell a dirt bike to make a little cash, I have had really goodluck in the past with 250 two strokes. I bought a 2006 rm250 for $1550, put $250 in to the bike to get it in running condition and then sold it for $2500. Now I am looking at a 2008 crf250r, and to be honest I have never bought or owned a four stroke. A friend of mine has the same year bike and I know it is a very solid bike. My question to you is when looking at a four stroke, what are some key points to look for on the bike? He told me the bike needs the carb adjusted(that could mean anything) and he told me it backfires quite a lot when running. So basically give me the run down on the good and the bad signs that can be noticed on 250fs, that way if I do go look at the bike I don't put myself in a hole and buy a POS. He's asking $1800 but I'm positive I can get him down to $1300-$1400 by pointing out the need of tires and a few other cosmetic parts here and there. I'll try and post a picture later for yall, but for now I am about to go riding