I went for an A3 1.8T sport which in my opinion is one of the most practicle and easy to tune cars out there. Standard engine is 150 and this can be remapped to 225, with an uprated turbo there are reports of 350 bhp on standard engine internals. The standard engine still gives me 35-37mpg round town!
This means that your power Modifications budget are just a remap

saving money for the visual Modifications.
The 1.8T engine also appears in the VW Golf and Seat Leon so there are some cheaper bases to work from.