youll struggle for 250bhp from a tdi mate, without uprating the turbo, and other parts.
tdi cupra is 150 standard, a good stage 1 map will produce a reliable 180-185 bhp, with a massive amount of torque, any higher torque and youre looking a an uprated clutch to handle it.
id concentrate on getting 180, which is immense in a diesel, and uprating handling to handle that power, then youll have a fantastic diesel powered macheni
price wise, look on autotrader and see what they go for, but i would say fsh, average mileage, £4000 ish