£2500 to buy a decent GTT, insure it, put money aside for the inevitable costs (basic servicing if nothing else), then have enough money to run it (fuel, tax, MOT etc) on £80 a week...
Even if you can get a decent GTT for £1250 (errr), how much do you think insurance will cost? Not much point going any further with this sentence.
Can't help but admire the inevitable choice you're about to make from the hopeless position you're in.
It wont work without it costing you more than you have but good luck anyway lad.
Tbh, if you're not afraid of a bit a debt then £2.5k isn't a bad starting point. You wont always be on apprentice wages and in any case, life is short.