Looney, I think you can forget a GT4 straight away due to the miles you'll be doing annually and also the (potential) running costs involved.
I'm not about to wade into the 6 vs 7 argument. What I will say is this: get onto Crewcutchuck, buy his Beams motor and stick it in your very clean looking, very nice 6!
[hypercatjohn]
If you install the Beams engine, nubile, eager young ladies will flock to your door. A bit like all the Lynx adverts, but smelling of burnt rubber! They'll all want to have sex with you (with your bonnet open, of course) and you'll never regret the decision.
FACT!
[/hypercatjohn]
It seems a bit silly to me to replace a car you appear to have poured a lot of time and effort (and money) into, simply because you want a little more power from it. Get the Beams, put it in. Job well and truly jobbed!