In reply to SlickDizzy:
Well I can't speak about the E30 having never owned one, but they are pretty sweet looking now later in life IMO. I know there is a huge following and parts are relatively easy to come by even now (correct me if I'm wrong here).
Now for Merkur time: That first 87 looks like a decent deal if it's stock as it looks (there's no intercooler it so that may mean it's been unmolested). It doesn't say anything about the A/C system, new shocks and struts are cool, ask what brand, KYBs are common cheap replacements, if they're Koni yellows or something silly they'd literally be worth 1/3 of the price of what he's asking.
Second XR on the list has an intercooler and has a miss, if he's replaced the whole ignition system including a new TFI module, dizzy, wires, plugs, and it's still got a miss it's suspect. The fact that he's got a spare set of injectors and hasn't botherd to do a simple resistance test on the ones in there makes me wonder if he's covering a burnt valve or other problem that's just alot of hassle.
Third XR seems decent as well cept the A/C.
Now for Merkur specific parts there are people who sell parts, Rapido, BAT, MerkurMidwest, MC2Racing, then the enthusiast community part them our regularly or have spares.
As for the Merkur itself:
Suspension, interesting design, full independent all around, once you see the front sway bar which acts as the second leg of the A-arm you'll appreciate how clever it is, that being said, no camber adjustment makes lowering more challenging (read requires modifications). The rear suspension is hard to describe, you have to see to understand, but it's all relatively simple to deal with.
Brakes: front discs, single pot caliper, rear drums, simple, effective, easy to upgrade, just need bigger rims
Wheels: 14" stock 'phone dials' suck, get rid of them, later 15" 'spatulas' are better, but finding once with the correct offset that won't rub is where it gets tough, but if you don't mind rolling fenders or removing inner splash guards, etc the sky's the limit. You can fit VERY big tires under the car with the right offset and backspacing, especially if you convert front struts to coilovers, see MC2Racing for inspiration.
Transmission: T9, sloppy, 2 piece drive shaft, guibo, if well kept they're servicable units with decent lifespan, if abused or put behind a lot of power won't last long, luckily a T5 swap is simple, easy, and converting to a single piece driveshaft is also a nice change (increases body vibration) you just need an adapter to the diff, 460Machine.com.
Diff: stock is open, suck. Luckily swapping in an Eaton, Supra, etc have been done and kits are available, see MC2Racing
Engine: Probably the most fun reason to get an XR4, many people will say the 2.3T is past it's prime and isn't worth anything, well it's an Iron block, Iron SOHC head 4 banger factory turbo'd from the 80s, of course it's past it's prime, it's also cheap and easy to mod, takes huge abuse, and is obscenely simple (non-interference). Stock longblocks can make big power, not as big as a 5.0 but a turbo's a turbo's a turbo.
Fuel System: craptastic 2 pump system (in tank + in line) replace with singe high pressure in-tank, injectors are 35#/hr 'brown tops' 200-250WHP is doable on them, upgrading injectors is easy and not terribly expensive and opens up as much as your motor can do, you can idle and drive nice with huge injectors on these motors, we have 75#/hr and have no problems, people say you have to run over 120#/hrs before you start having problems...fuel tanks like to rust, can mod a newer mustang to fit 
EFI: GOD I HATE TFI, but when it works it works good, when it doesn't well, you learn what to check, what tools and spares to carry, and how to diagnose it on the fly. Dizzy + TFI module = spark, computers can be swapped out, repinned between SVOs, TurboCoupes, etc, some can be chipped, or you can open up potential with a MegaSquirt
Intake: 4 single long intake runners, upper and lower, gut upper, port lower, your engine will thank you
Exhaust: Cast iron log header to T3 flange (make sure the support bracket is around or you will crack the header) Oh, bigger is better here, 3" is the way to go, though you'll have to step up turbos to make use of a monster exhaust like that.
Turbo: T3 (can't remember the A/R) but they can be swapped, hybridized, across XR4s, SVOs, TurboCoupes. You can also do crazy things like put Holsets on them for crazy boost we've had ours over 30psi, T3s usually top out around 18-20psi (15 is stock for a manual trans car). Get a manual boost controller and turn it up, computer and injectors can handle it, but you gotta watch it
Headgaskets: you will pop one if you turn it up, don't fret, just know it's going to happen, then when it does you won't be upset.
Intercooler: stock there isn't one, add one if you turn up the boost, those two mods can literally take you over 200WHP and cost under $200 depending where you source your IC and piping, couplers and clamps (tbolts from ebay + Semi Truck Heater Hose makes great cheap couplers)
Battery/Wiring: You're on your own here, know that 80s wiring from Germany sucks, period, having a battery sit 3" above a hot turbo/downpipe isn't exactly the smartest choice, relocate it, watch out for bad connectors, chaffed wiring, bad insulation, etc, and REPLACE IT ASAP unless you like Carbeque
Interrior: Can't help you, I've never seen a real interrior of an XR4 except in pictures, ours came stripped 
I wish you luck, oh and if you want to know more about XR4s go to TurboFord, MerkurSport, MerkurClub, MerkurEncyclopedia, just be warned, you may get the bug if you read to far into a 2.3T powered rig, I know I did, can you tell?
