The first and most obvious thing, is know what you're getting yourself into:
480 Common faults - "Oh they all do that...."
- Pop up motors failing
- Front DRL lenses cracking
- Rear Lights Fading/cracking
- Blocked scuttle drains.
- Damp interior
- Door stops cracking/fracturing
- Dashboard cracking on Upperleft corner
- Speedo failing
- Infocentre Failing
- Rear Arch rot
- Rear bumper support rot
- Did we mention damp?
Whilst its very easy to say "buy the best car", we're at a point where the good ones have good owners, who aren't going to let them go, and everything else needs work to some degree or another. And what if you wanted a turbo? With fewer than 50 cars remaining, it's more a case of buy first, worry later!
What to do next
Start by assessing the car; What works, what doesn't. What bits of unobtainium are you going to need? Check every electrical component, see if it works. Take lots of photos at this stage, so you've things to refer back to. Writing a list at this stage gives you something to work against, lets you prioritise stuff. It's this prioritising that will keep you sane. Whilst it's easy to say you're going to do a "full restoration", ask anyone on here that's done one, and you can say goodbye to a couple of years of not having a car to drive, which is a massive motivation sapper. Instead, focus on a rolling restoration. Getting it MOT'd and using the car will give you more things to fix, but at the same time, let you enjoy the car.
Interior OUT - All of it: Seats, carpet, door cards. The more you can remove the easier it'll be to dry. Not only that, you can start to see quite easily WHERE the water is getting in and thus, start to remedy that. It also means you see some of the inner structures of things like wheel arches and doors. You can assess the rust, or if you're having to replace any with fresh metal, a removed interior has far less chance of catching fire than one still in the car! After welding, its very easy to protect those surfaces from future problems with rust killer/cavity wax has you have far greater access.
Clean it:Another reason for removing the interior. Because when you clean it, it WILL leak (have we mentioned the damp problem?). Remove the Upper outer trims as a horrible amount of dirt will collect under there, Give the whole car a damn good soaking in Traffic Film Remover and let it soak in, before pressure washing off. Get under the rear arches, particularly around the fuel filler neck area, as muck will get trapped here. Remove the front plastic arch liners, as the amount of dirt that collects at the base of the front wing is terrifying. You can blast it from the "door side" but you'll never be sure you've got it all out with the liner in place. I actually prefer to have the car on axle stands and the wheels off at this point, mainly so the wheels themselves can get a good blasting around the back, but it also gives you great access to the arches. Also check your drainage points. Not just the classic "under bonnet" drains (they will be blocked), but the front and rear of the doors, the back corner drains (that will be blocked thanks to the evil sponges) and the sill drains.
If you're super brave like me, you'll have no problem sticking the jet washer INSIDE the arches and doors - That's right, spraying water inside the car! Doing this will make sure that water can make it's way OUT of the car as Volvo intended. When dirt and grime block the exits, that's when water starts to find other paths to get out, which includes rusting it's way out. You may also want to attack the engine bay (if you're a bit more cautious, you can cover the alternator, distributor and coil). Clean engine bays make oil leak tracing easier as well as making it a nicer place to work in. there's also a scary amount of places that grime, and therefore rot, can build up.
From here, its about putting the list of jobs into some sort of manageable order. Make a plan, try to stick to it (Do as I say, not as I do!). For example, getting it through the MOT should be an obvious goal. Now the MOT won't care if your paint is mismatched, your carpet is ripped, or it's leaking like a seive, so it's about prioritising jobs. I tend to leave things like brakes and tyres until before the MOT, simply because having them standing around not being used isn't the best for them. Typically I go engine, suspension, brakes, wheels, MOT.
Once you've got an MOT, you can now focus on the usability items. These are the things that would stop you using the car as a daily driver. Leaks, rattles, electrical things that don't work are all usual suspects.