I have pretty much the same as you, 1.8 k, roughly that hp, 3.9 diff, 13" rears with 205 cr500 tyres and 6 speed. I find it fine, acceleration is very good, probably cuts top end a bit and I suppose it's a bit revvy on the motorway. On the roads round my way and on track it's great though. That said a £1500 6 speed maybe fine or it may need £500 more spent. Hard to tell. A tracsport setup would be the best of both I expect as it's 4 fairly close gears then an overdrive when needed. I'd get one of those built into your box rather than risking a used unknown 6 speed tbh.