Based on my 3-4 years of recent vintage volvo (amazon) experience BODY condition should be your primary concern. Completeness is second, followed by engine condition. Volvo used the same (or similar) engines from the early 1960s through 1975. It is much easier to find a new engine and drivetrain than to find the proper piece of chrome for your dash.

The likelihood that the car is truly rust-free strongly depends on the car's history. About the only rust-free cars you will find are from California or maybe sat in a protected garage for Much of the last 40-50 years. You're in Maryland? If the car's from that area too - expect rust.

My 68 was a garage queen by Missouri standards and spent 20 years in a barn. It still has rust behind one headlight and a fist-sized hole in the passenger floor pan. It also had an amazing ammount of rust in the fuel tank. I cleaned it, but when the pickup tube snapped off, I just replaced the whole thing with a good used one from Chris Horn.

The others are quite right to tell you where to check for rust. A really fine looking car may harbor some serious ugly under the paint.

Case in point. My 1973 1800ES.
It looked like this on the outside...


But like this under the bondo-ed rocker panels.

I also think the frame may have been bent.

If the car really looks nice, I'd bet dollars to doughnuts it has some serious issues hiding under the skin.