If you can find a dealer who has one reasonably priced (~$125), just get the battery from the dealer. It is a good unit; get one rated at 80 Ah. It displays european specs, so it won't list CA and CCA. So make sure you see the 80 Ah. If that number is lower, it will be either one for a NB (a little less - in the 70s) or a gasser. If they show you the gasser battery, just insist on one for a 2000-2003 Golf or Jetta TDI. Insist. If they act like you don't know what you are asking about or they claim not to have one, walk out and tell them they are a disservice to customers like you.
Then you'll be stuck in the world built for other cars, but not quite yours. The battery you have now will be OK in the summer, but it will be iffy in the winter. During a cold snap you may have severe difficulties running the glow plugs sufficiently and then cranking the engine fast enough to start. So get as many CCAs as you can - I'd go for more than 900 if at all possible.
Then do a search to see what case you need. IIRC (and that is suspect these days) a 94 will work, as should a 48, but the 48 will be too low in capacity most likely.
Good luck!