We need a FAQ on buying 1800's.

The 3 most important things to look for (IMO) on an 1800 are:
1) Rust
2) Accident damage
3) Rust

They have complicated body structures and repair shops seem unable to do a good job in replacing panels so accident repairs often don't look right. Rust will attack the underlying structures as well as the visible sheetmetal, and can be exceedingly hard to rectify, if you can't do such work yourself then the labor costs involved make paying more for a solid car the cheaper way to go.

Mechanically they are pretty bulletproof. A standard Volvo drivetrain with few weak spots (cams and cam gears perhaps) but nothing that can't be easily and (relatively) cheaply put back to as new. I would avoid an automatic transmission as it makes the cars as slow as a school bus and they also tend to stop working if driven spriritedly.

Interior pieces can all be had now, so that isn't a huge concern either.

I'd place the value of the car roughly as follows:
Body - 75%
Mechanical - 15%
Interior - 10%

Places to look for rust:
1) Sills
2) Crossmember that runs under the front floorboards - and contains the fornt jackpoint
3) Bottom edge of front and rear fenders
4) Floorboards