I agree, you should get it checked out before you make an offer. The spec for idle speed is around 700 rpm, if I recall. I can get mine to idle smoothly down to about 550 rpm, but everything has to be perfect. There are several potential causes for the rough/slow idle, and there is no way of determining what's going on with out having it inspected. It's worth a couple of $100 to have it checked out if you are seriously considering it.