If I was selling in 3-4 months, I wouldn't be thinking of shelling out for four new 20" tyres. At the upper end (admittedly), 4 Michelin PS3s would cost upward of a grand, which is not far short of 50% of the value of the car. You'd not get half of that investment back again - so that would effectively be costing you c.£200/month for the tyres.
I'd get 4 (good) part-worns from DM tyres (see tyre thread..), and run on them - they wouldn't take much work to look reasonably new - and when you come to sell, I'm not sure the buyer would see much difference.
Sorry if that sounds a bit like a tight Scotsman
, but I'd be saving my pennies to invest in my next car, which you'll get more of a return on (assuming you keep it for a while).
However, the above is just a personal opinion, and - having just shelled out £3k on my gearbox - I admit that logic doesn't always come into it
