to be fair james, your not making it as easy as i thought you would!

the alfa sounds good that the kit comes with brembos and a sus set up. but you can easily put far better brakes than that on the GTI if you budget for it too. GTI comes with "sport suspension" too but id imagine as with both cars you will want to do a little something about that. im sure it has good torque but remember its a diesel! short shifting while hamming round corners/ lanes/ tracks you will get bored of id imagine. 19s do look good but as you say come at a price, ride quality will be reduced and you will be spending lots of wonga on tyres. residuals are a bit poo on the alfas, then theres the build quality! i think the GTI has far more to offer as a fun, lighter more comfortable place to be. and obviously the GTI or ED30 would be far faster and probably not far off on the economy too.
i havent driven either of theses cars but id imagine the Z4 a hoot if you like chopping hair and smoking menthols! Theres a guy S08 on the other forum who decided to trade in his ED30 for a Z4M and regretted it! the GTI or ED30 would obviously be the way forward. dont rule out the standard GTI as there are a couple of really fast ones about, i think bacillus who frequents here has a bit of a speed demon as the dynos have prooved and so does topcat from the results of his 0-60 and quater mile times.
if it was my 15k, i know what i would do get the best ED30 you can then start saving for mods. or you could pick up a very nice GTI and get all the mods done straight away.

id imagine there will be some movement in the budget so put a bit spare to get yourself a map. as i said do your research, i did mine for months and met lots of people, went out in cars and spoke to tuners then went revo

stage 1 Revo on an Ed30 or GTI is a revelation. stage 2 was pretty good but then stage 2+ is blooming awesome!!

edit to add, i agree with what iwilkins has said, obviously! but it is an understated yet beautiful car.
bare inmind if you do want to modify that the standard clutches arent all that strong, so you would have to budget for an uprated clutch if you went for an ED30 with mods/ map. DSG has prooved to be very strong as Hurdy will verify when he gets back from holiday
