Hi there
Being honest, £50 isn't going to get you very far with respect to improving her paintwork significantly, especially if it's as neglected as it sounds. If you have little or no experience of paintwork correction then the best (but pricy) solution is to get a professional detailer to sort it.
However, it might sound obvious but maybe starting off by giving her a thorough hand wash (2 bucket method), dry and getting some protection on - sealant, wax or both, will be a good start (if you haven't already). Sometimes it's amazing what a decent wash can achieve. No it won't alter the fact there are stonechips and swirls but at least you will be able to make a proper assessment and have a plan to work to.
Having the proper gear will make all the difference and getting together a decent basic washing/drying kit is all you will get for £50, in fact probably closer to £70.
There are a few threads in this section including the 'detaing guides and FAQ' subsection, giving basic washing advice and more - here's a couple below. Although some relates to a newer car, the advice for washing and protecting are the same regardless. Some of this may well be irrelevant but worth a look:
http://www.golfgtiforum.co.uk/index.php?topic=273081.msg2498291#msg2498291http://www.golfgtiforum.co.uk/index.php?topic=274098.0However for proper paint correction/stonechip repair, if doing it yourself you would need to invest in the right gear such as a DA polisher, pads and polish compound, and by the time you've costed that up, getting a professional detail could well work out cheaper. You could try and polish out some of the defects by hand but if swirls and marring are medium to bad, you will need a machine. Hand polishing can really only offer limited results.
If she's in quite bad a state, save up for a detail and suffer the Mrs' fury.
Hope this is of some help.
Rebecca