With £20k you could handpick a 996 for that money. Go look on PH, the market is awash with them and their prices are in freefall. Choose wisely though, there are alot of lemons out there. Don't forget though, this was a car that cost £60k new, and with it comes maintainence costs. As said by Gazboy, the RMS is prone to failure on the 3.6 and there were cylinder liner issues on the earlier 3.4 and when they go wrong will cost you an arm & leg to fix.Provenance is everything with these cars, and if it hasn't got the right stamps in the book or been serviced by an OPC or respected specialist, then you could end up being stuck with it.
Personally I would look at buying either a 993 or a 964, their prices are holding nicely. If I'd kept my 993, it would have been worth more now than when I bought it 5 years ago.