I think you should buy Ian's supercharger kit and fit that. You've even got his build thread on here for a guide! Then all it would need is a tune.
Supercharged Zeds are awesome and will give you enough power to be getting on with for a fast road/ track car.
A TT would obviously have larger potential but would be far more expensive to do, and not really a first job for a novice.
That should keep you happy for a while or until the 370 comes down in price. Then sell your S/C kit (you would get most of your money back on it) sell ya 350 and get your 370 if it's what you still want. But you might change your mind by then.
If the work scares you, sell yours and buy one already done like Steady Zeddy's, very good value for money.
Or if you are happy with your car, pay the loan back. Its a nice thing to be debt free. Then either save up your wage increase for something else (370) or mod yours bit by bit.
Its a nice dilemma to be be in.