In a previous life I'd get a GTI, never a new one due to expense (no PCP's in those days) and start to modify it almost as soon as I'd got it. The one thing I learnt over the years was that whatever you modify you gain in one area and lose in another.
The mk1 and mk2(8v) GTI's were torquey but lacked top end - so you modify the cylinder head and realise the the bottom end torque isn't so good... What do you do next? oh yes, you then have to put a 1900 or 2000 bottom end on it at huge expense then wonder why you didn't just get a 325i instead.
The suspension looked a bit high too, oh, I'll chuck a very expensive German kit on it. Then the ride is ruined. And so it goes on.
I've modified TDI's too. The GTI's got a bit boring (understatement) in the 90's so I thought I'd try something else with these new fangled TDI's just released. Add tuning box and yay! Suddenly it's quicker than a GTI of the time and more fun to boot with all that torque; ok, I'll make it faster still and add a remap and injectors and suddenly the fuel economy has gone to poop! Oh, the suspension can't handle the new found power, I'll upgrade that too - now the ride has also gone to poop... The brakes? I'll leave it there!
The best thing that ever happened to the sporty VW's was when the GT and above spec TDI's and turbo'd GTI's came with proper sport seats and engines that you could just add a stage 1 tune to and use reliably without going silly on the mods. With the mk5 came proper suspension too, and by then there are so many electronic luxuries added to the cars, coupled with all the emissions crap, that you'd not really want to run one too far after the warranty ran out.
I've lost count of the amount of people on the forums that have bought their GTIs and TDI's and modified them so far that they'd lost sight of why they bought them in the first place, and still never found that utopia they were looking for. If you want a sports car then buy a sports car is my considered opinion after about 30 years of messing about with GTIs and TDIs. Buy a cheap sports car for fun and a GTI, or better still a GTD, and just appreciate each for what they are.
Alloy wheels have always been my weakness and trademark, but the mk7 is a hard one to choose wheels for that actually look good. Especially if you want non diamond cut wheels to avoid the inevitable problems. I've changed my wheels twice on my mk7 already and was looking for yet another set before running into a brick wall of options. I appreciate everyone's tastes are different but with a Golf you want it to keep it's classy classless looks but at the same time you want it to look a bit different to someone's leased company car. As said above - you want to put your own stamp on the car.
Mr N, I'm not sure what your hobbies are but I'll always argue that if you play golf or have a football or rugby, cricket or whatever season ticket that all costs money. If you collect stamps or coins it costs money. If you buy fine arts then it costs loads of money. If you buy a holiday cottage because you're particularly flush then that too will cost money, and lots of it too!
If your car is your main interest then you can justify spending a bit of 'dead' hobby money on it and only you can know where you'd be prepared to draw the line.
For me, in your shoes I'd do the mods you have already done as they're all classy, subtle and pretty cool. I'd maybe look at a different set of wheels but as the GTD comes with the best looking wheels in the range I'd probably not bother - but I would add a set of spacers front and rear to give the car the sporty shoulders it lacks (all Golfs look better with a set of spacers but the mk7 benefits the most since the mk3). I'd add a tuning box (I wouldn't bother with a remap on a TDI as they work differently to petrol engines and I like the fact the DTUK ones are adjustable to suit your mood) which will still have some value when you come to change car unlike a remap.
At that point I'd call it a day and concentrate on saving for my next car which will have lots more gadgets. And with technology gaining pace all the time I'd just appreciate that whatever gadgets were on my next car they too would be out of date after 12 months...
