Thermostat - common failure.
Radiator / water pump, due too bad post crash repair.
Front TCA bushes and front suspension bushes, but since uprating to polybushes, no complains. (highly recommended!)
Core plug blew on 2nd hand engine, but was replaced. (think water pump prob as well)
Exhaust, non stainless, was changed to mongoose - 100% better :) (nice noise)
Fusebox can cause probs, if lid not closed properly.
Read diff failed, was rebuilt, and has been great ever since!
Gear selector plastic thingy broke, whilst in outside lane!, fixed and fine ever since.
Great car, cheap and easy to repair, modify, fun to drive, very responsive!
Only problem came when I lowered the whole car, then wanted to add a stainless exhaust. As it fouled when going over speed humps, so raised the rear suspension with uprated springs, and has been fine ever since.
The main problem was, as is fairly common, the DOHC engine, I've had 3 in the same car, but since the crash it's not been the same : (
The car is now off the road, waiting for extensive repair, or stripping for parts, but have had several sierras before, including a 1.8 saff, 2.0 xr4xr (not bad), a 2.9xr4x4 1990 (nice and quick ;-) and a 1.6 for spares.
The interior is a dream to work on, Haynes manuals are very informative and easy to follow, as are the wiring diagrams (added several extras that weren't standard on the GT).
Possibly the only complaint with the car, it wasn't fast enough!
Shame it's not made anymore.
Parts are still available, but, and I stress this, Ford won't keep making them forever, as I've found out a few times.
Funny bit - in the past, I used to pass the next reg up, exactly the same car, model and colour, on my way to work. We always used to flash each other!!