Hey, welcome to the site. Good to see another non-GTI member on here. If you're a student you'll find a lot of mods on here are pretty expensive. There are quite a few things you can do that will help spruce up your car.
Wheels - I'd recommend maximum 17 inch alloys, preferably made by VW. There are several styles out there from the Golf. Look up Monza 17's and Classix wheels. These can be picked up from as little as £150-200 with decent tyres.
Colour-Coding Body Parts - if you know your way around painting then its a relatively simple job to remove your door handles, prep them and paint. Door rub strips will need to be done on the car. You'd need to mask up the rest of the door and be very careful not to get overspray on the rest of the car. If your mirrors aren't colour coded then they would look nice done too. These are a bit of a pain to dismantle but you'll probably find a How-To Guide on here or Google.
Headlights - GTI's come standard with black insert headlights. So do some late GT models, some come with grey insert headlights. I haven't seen your can but I'm guessing it will have chrome headlights. If you keep you eye out here or on ebay you can pick up a set of black or grey insert headlights that'll totally change the front end of your car.
Headlight bulbs - Dim, yellow headlights don't give off that sporty look. I'm currently using Philips DiamondVision H& Bulbs which are great (very white) but as always there are loads of other alternatives.
Audio - VW offer a few good stereo's. Your car will probably come with the RCD300. A good upgrade would be either a RCD310, RCD510 or RNS510. These range from £50ish (RCD310) to £500+ (RNS510)!
There are other brands/makes of stereo but I haven't tried anything else in my Mk5.
Bodywork - The valance from a GTI is relatively cheap £50ish from a VW Dealer and if you get a good bodyshop they shouldn't charge you the earth to paint it. If you get it painted with all your other non-colour coded parts they may give you a good deal. Look around at a lot of cars on here that have half the rear valance painted. Looks v. good in my opinion.
Lowering - You're best going for lowering springs as opposed to coilovers if you are on a budget. £150 might get you a cheap set of coilovers but would get you a top quality pair of springs. Look into Eibach or H&R Springs for the 1.4 model.
Engine - A remap will do nothing for your car and you'd be better saving that money for a newer/faster
car Golf. All cars would benefit from a good service including changing the fluids and a good quality air filter. I'm not sure what there is on offer in the way of aftermarket 1.4 filters.
The main thing that makes a lot of cars stand out on this forum are the little mods that on their own don't seem to make much difference. But add all the little mods up and you find yourself looking at a pretty decent car.
I hope you manage to get your car looking how you want. You'll find a lot of useful info on here (part numbers etc). Remeber to show us pictures though!
PM me if you have an idea of what wheels you want.
