I have a stock 2003 300ex which needs new rear tires and i'm going to trade it in and get around $2800 or less (I paid $2700 for it) and get an 2003 400ex that looks brand new and has yoshimura full exhaust and a front bumper and maybe some other stuff i can't see for around $3800

 

Sounds like for $1000 that is a good step up if the 400 is in as good mechanical condition inside as it looks on the outside.

If you are getting it from a dealer:

1) Do they offer any warranty on it at all - like 30 days so you can make sure you are not getting someone else's problem they were able to pass off?
2) Can you find out who the previous owner is and talk to them?

 

ive seen 2001s go for 2300 on ebay, so if you sell for $2800 then you will have 500 bucks to customize the 400ex, i would get a pipe