Welcome onboard Nick, you certainly WILL get best of both worlds with the SEAT Leon FR (TDi 170).
This is what I own, excellent performance (considering its a Diesel) and returning over 500 miles per tank of fuel, at 50+ mpg !!
You will comfortably find an 07+ 170 TDi FR around the price range that you specify, with a similar mileage. Also note that cars with the optional Bluetooth package, also feature a larger multi-function dashboard display - something to look out for.
You can still Remap this car, and comfortably see over 200bhp (and importantly a large increase in Torque) at a reasonable cost, should you go down this route.
I'm not aware of the FR coming with the (BTCC) Aero Kit, however this may have changed with the current "facelift"
Mk2 Leon FR (Diesel), which also see's a Paddle Shift gearbox as an option.
As stated previously, some fit the BTCC Kit side skirts and rear spoiler to the FR, which is a great addition - depends how far you want to take that look !