Hello, welcome and good luck.
There are others far more qualified to answer the specifics in running a Diesel Golf but I will say this,
avoid the 1.4 TSI unless going for the turbo only low powered model.
The 1.4 twin charged unit is going to be a nightmare when in warranty and a mill stone round your neck when the warranty finishes. Join seatcupra.net and have a look at the mk5 Ibiza section to see the problems people have been having with these amazing engines. VW have modified lots of bits but I wouldn't trust one that is going to go past the warranty period. Admittedly this doesn't apply to every engine, and there have been many that have had no problems. But I wouldn't trust one with my own money.
On the other hand the 1.4 TSI is often offered with a few discounts, but to me this can be worrying too as VW don't discount something unless it is a very slow seller!!
VW aircon is no more troublesome than any other aircon. It needs to be used regularly to keep all the seals doing their job properly. The GTD somes with fancy automatic dual climate aircon which has lots of parts extra to the basic aircon. So potentially lots to go wrong. Most people prefer the dual climate thingy but I prefer the standard aircon.
A lot of mk5 Golfs suffered aircon compressor issues but I think they are a reasonably cheap fix when out of warranty as long as you go to a decent aftermarket specialist. Having said that I sold my mk5 when the aircon started playing up!!
