Well...I can give you my recommendation as to what I would do to get the most for your dollar(US dollar because I'm a transplant to the US from England, Scotland, and Ireland at about 1700ad
) I did the same thing...tons of consideration on what to get for it to perform well while spending the least I could so as to not be replacing things because of my choices not working out. Just did springs @ 22k and the rest @ 58k miles
Control arms - S3 parts? Cast iron parts? Or leave stock?I wouldn't do this...but I would swap out bushings when needed and get a Whiteline anti lift kit(W.A.L.K.) first.
Droplinks - ECP or VW (OEM)? Are there any upgrades?There are some and they are usually adjustable. I lowered mine quite a bit and kept the oem links figuring they can get upgraded if the oem fail to last much following the drop. The car was lowered @ 22k miles and I am @ 132k now. None of my links have ever broke or bent...so I would use oem again should they fail after all this time. I drive aggressively/playfully when safe or at the track and have been at its limits often.
Polybushes - SuperproThis, as I said above, would only do if I had to. I check the tightness of mine every time it is on the lift and they are still tight. I would likely get oem if needed because of how well they have been.
Dampers + Springs (Really struggling to decide which one to go for here)Eibach sportlines are a good drop. I now have a pair of H&R SuperSports in front because they are a bit lower in the front than the Eibachs were, but mostly they are comparable in drop...but the Eibachs were more comfortable. I also have a pair of adjusters in the rear with a pair of H&R Street performance coilover springs but they are paired with the B8's. I would get Bilstein B8's to pair with whichever springs you choose unless you are considering coils. If you don't want the B8's...you can save a buck and choose Koni yellows. I am switching to H&R Ultra-Low coilovers if I ever need to replace any part of these in the future. I rode in a MK5 with them and shocked how comfortable they are.
Ball joints (One of the bolts snapped recently) VW? ECP? Upgrades available?Stick with oem.
Top mounts - RS3 parts?Again...if oem doesn't fail often, or at all, stick with oem.
Any other parts?Best additional bit would be a rear ARB. I have both but I had just a rear ARB alone for some time. It dramatically eliminates understeer when set at its stiffest position. However, with just the rear ARB...I could feel the axial body twist, from front to back, having the rear stiffened only. So I added the front ARB and set it on the lightest setting and that keeps the front end from axial torque about its length.
The car currently...
When it just had the Eibach Sportlines