Just to be nit-picky, a '68 is model 1800S. P1800 only refers to the first 6000 cars built for Volvo by Jensen.

The original motor would be a B18B, with 115 HP and 10.5:1 compression. See if the block has B18 or B20 cast in. Dual SU HS6 carbs were standard.

You're looking at a 35-year-old car... unless it's been completely restored recently (and it'll be priced accordingly, you can be sure), expect problems. Mechanical problems are relatively inexpensive to resolve. Interiors can be redone. Rust is very expensive to fix, as is replacing or rechroming trim. Look under the car, look under the carpets, look at the rocker panels.