IMO - Buy the best body shell you can - never mind the paint work as if you are like me - it will be getting a respray anyway (Mine was done 18 months ago & now in getting the final bits done - roof & bumpers), then gradually replace all the parts that you need to as & when you find better than what you already have....
I buy most NEW Body or Interior parts via the dealers - (I use a guy who works for TPS & can get up to 30% off)... Or from VW Classic in Germany - I have also used VW Heritage for a brand new MFA stalk as I could not get one from either of my other 2 sources...
It's worth noting that UK dealers have different stock to what VW classic have in Germany - I have tried to order a sunroof seal from Germany and they didn't have any, so tried VW - TPS and they have loads - But as usual the prices from dealers are expensive... £45.60 plus VAT...
Running gear stuff is 10 a penny and as already said there a plenty of OE suppliers out there... so nothing to worry about as such for keeping the old girl going...
HTH
My re-build thread may help you -
http://www.golfgtiforum.co.uk/index.php?topic=140182