1) Does anyone have a recommendation for any garages in Essex that specialise in Impreza rebuild and repair
Not London based but I’d highly recommend TDR Warwick; engine rebuilds, maintenance, remap ect my go to if ever I need anything.
https://www.tdracing.co.uk
2) What parts are recommended to be replaced at what mileage?
Belts every 50-60k probably more but if you’re doing upgrades then everything
3) How many miles between oil changes?
I do 4,000 miles only use genuine Subaru oil filter and if over 100k I’d recommend running 10W40 (can maybe stretch to 6-10,000)
4) What should we be looking for when we go to visit the car, coloured exhaust smoke?
Rust on rear arches is a common issue, Check oil is clean, not gritty when rubbing oil between fingers via dipstick, if lean and de-cat it will spit out carbon buildup (soot) on a cold start normal, white smoke on cold start and should evaporate fairly quickly. Sound of any knocking from engine bay piston slap or dreaded rod knock.