122's are rather over built. It's not going to fall apart. I've seen 122's with EPIC amounts of rust still apparently structurally sound and driving.

That sort of patching is perhaps the easiest body work you'll ever come across. Simply because it's underneath the car, no one ever sees it, and it doesn't really have to look good (well, unless you are vying for 100% scroes in some really competitive concours type event!).

Floorboards are quick to go on old Volvo's, usually due to the rubber floor mats they used. They kept moisture off the metal for the first 6 months, then held whatever finally got under it there for the next 40 years. But it's a sort of leading indicator. As are those crossmembers. But since those indicate exposure to rusting conditions, I'd carefully check other rust prone areas to see if they've been poorly patched in the past. Like the front fenders around the headlights and down the back edge. The sills. Around the rear wheel arches. You might find rust starting and be able to stop it before it really gets going.
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'63 PV544 rat rod, '93 Classic 245 + turbo