I've never owned one of these beauts, but have done considerable research on 'em as I am always keeping my eyes open.

Disc brakes were introduced in the 1970 model year. Prior to that, you're looking at discs up front and drums in the rear.

Automatics were 3spd, and standards were 4spd+OD (M41). I believe, but am not absolutely sure, that all sticks came with overdrive.

The original motor was a B18B (1.8L) breathing through dual SU's. The B20B (2.0L) was introduced in 1969 and beginning in 1970 all Volvo models had Bosch fuel injection. These are the most powerful of the B20 engines (B20E), rated at 130hp. This continued until smog regulations took effect a few years later. The B20F was a lower compression motor that met the smog standards and was rated at approximately 115hp. I think that you will find this motor under the hood of '72-on models (i.e. 1800es).

Apparently it is very difficult and expensive to properly repair rust/body damage on an 1800. They are said to never look "quite right." Has something to do with the way that the chassis was constructed, I'm told. So your best bet would be to find one with minimal (or no) rust. Perhaps a California car.