I've had a couple Volvo p1800s, both 72s with fuel injection. They are great cars, but (like mid century Benzes) they rust like crazy, particularly the rockers, cowl, rear fender wells/shock mounts and around the headlights.
I might even still have some parts laying around.
Mechanically, I have to say that they would stand up with any Mercedes. The B23 straight six is a bulletproof workhorse, originally designed as a tractor motor if I'm not mistaken. I used to subscribe to SEES (the magazine of the Volvo Club Of America) and there were many 1800's reportedly having 4-500K+ with original motors and [manual] trannies.
I would love to have another one, not the 73 wagon though, I never really cared for the wagon.