All the 6-Cylinder engines in the E30 were good. They had less work to do than in the 5-Series owing to the weight of the car, and the build quality was excellent. The odd head gasket on the 320 and the 323i (which they only did for a couple of years), but the 325i is bulletproof.
The 1986-1988 325i Sport was a great car - it even had a Limited Slip Differential, which made the tail even more fun! 1989 saw the revised 325i Sport with more of a bodykit and revised interior. As these got older and cheaper, the boy racers got hold of them and nice ones are sadly few and far between.
Older, more 'mature' people tended to go for the 325i in standard form, and this is where the lower mileage and better cared for examples are to be found. I'd buy a 1988-1990 'facelift' model (plastic bumpers, not the chrome ones on the earlier versions and also revised interior and lights) with anything up to 80,000 miles and a service history showing regular oil changes.
Colours - Zinnober Red, Diamond Black and Dolphin Grey were the most popular - but i'd go for a metallic as the solid colours will have faded badly by now if not looked after meticulously. The lacquer on the metallics was really thick and kept the shine beautifully for years.
Auto or Manual? Either - the gearbox was never strained. No issues to report either.
Things to look out for? Well - apart from the obvious, the E30 really lets you know when it's tired. Diffs whine, the gearbox linkage feels sloppy and the seat will be collapsed and/or frayed on the centre and/or bolsters. If it looks tired, it will be tired - walk away.. there will be others.
Lee
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