Welcome on board JonnyB!!
What price range are you looking at?
They are many Mk5 variants to choose from, the GTI, Edition 30, Pirelli and R32 and not forgetting the TDI's. All have their plus points. If you plan on modifying your future Golf, you might consider an ED30 or Pirelli with their k04 turbo's.
Personally would not say there is a "best spec", just
your best spec, do yourself a "must have" list, example being 3dr, DSG, Xenons, MFSW in Tornado Red (BTW, the fastest colour)!! Get yourself the latest model you can with the lowest mileage and happy days!!
When you've blown your
WHOLE budget on the car, it's time to mod it!!!
Cheers