RE: part numbers..
Where are you?
If you are in the US, try the sellers above, if you are out of the US, they might ship. Might be more shipping, but, hey, it's an Audi,right?
To go by numbers, 1, Why do you need a front strut assembly? I don't really know if the 4000 is so much different than the 5000 that they had a non-renewable front strut. On the 5000, you unscrew the cap, pull out the cartridge, like a shock with no mounting flanges, drop it in the tube, reassemble to the inner fender.
2. Lower Control Arms? Available. Something like 95 USD, per. You get the whole forging, with new inner bushings as well as a new balljoint, just did thar. 4000 might be different, as the wrecking yard I went to ordered parts for a 90, and way different than a 100. If yours is a replaceable ball joint, should be cheaper than mine was.
3. I don't know what a balance rod is, sorry.
4. Steering rack, mebbe a couple hundred for a remanufactured piece. Not fun to change out. Can be done. If you are in Europe, I understand you CAN buy parts to repair it yourself. Mainly the piston seals leak, then bleed too much fluidto the rod seals. Mainly, if I understand it correctly, because the prior owner topped it off with ATF, which, I have been assured is a no-no, with Audi, myou use Pentosin (SP?), period, or mineral oil from the local drug store in a pinch.
5. Calipers, I don't know. Why do you need them? Remove them, follow the directions in the book, and the repair kit, rebuild them yourself.
6. Rotors can be bought, or, if yours are thicker than the "min" cast into them, resurfaced.
7. Front hoses, again, aftermarket item. SJM?
8. Is the '87 front disc, rear drum? Local auto parts could get those, no prob. Or, SJM, again.
9. I don't know what "brake liners" are. Do you mean lines? I bought one from the local NAPA store. The rubber hose to the rear caliper. Broke, put a ball bearing in the fitting to be able to drive till the hose came in. It did shortly come in. Still have it. Gave up on the car before I installed it. If you mean "lining", ie, "shoes", again, auto parts store.
10.Booster and MC assembly, 1 may be bad, and the other may be bad, but I doubt both would go as a unit. Boneyard, best bet, unless you have a ton of money. Here, in the US, there are places that rebuild boosters. High priced, though.
11. Rear dampers? Rear shock absorbers? Should be available world wide. Just go to the store, if there is one. Go online if you are in a "quote" 3rd world country. No offence intended, have no idea where you are. Sri Lanka?
12. On my '85, with rear disks, they were rubberised hoses, not solid tubing, so mebbe you need hoses. I got mine from, again, the NAPA store.
13. I just a few months ago bought 2 Remanufactured front drive axles for my '90 Model 100. 49.99 per each, 60 USD core charge, return the takeoffs, get the 60 bucks back.
Entire axle, inner joint, new boot, outer joint, new boot, new nut. Reground, larger outer race, smaller inner race, larger *****. Guaranteed for a year. Better than the ones I had replaced. Locked tight, those, at least the right one was. Some will not buy reman. BFD. I won't pay 250 + for OEM. Simple.
14. Brake pads are available at almost all the parts stores. May not be in stock, but most likely 2 day delivery.
15. Engine mounts. Again, SJM. Give them a try. Also try Rock Auto and the others in posts above. They seem to have more parts, and ship sooner, than the other vendors who advertise here.
16.High pressure hose to the PS box. Mine got a pinhole in it, pissed a whole lot of fluid out, real quick. Bought a new one, the car died before I installed it. Banjo fittings, both ends.Hole with the hollow bolt in it. Got one for an 85 5000, don't know if it is the same for your car.
Anyhow, could you possibly need all these parts and repairs at the same time? Seems unreasonable, to me.
If you are looking to repair anything that COULD go wrong, come out of your bomb shelter, because the biggie will get you any way.