No such thing as cheap when your in a golf haha. Interior wise you could look at a nice OEM VW touchscreen RCD510 headunit. Nice bit of kit to have in the car and a bit of an upgrade from the standard RCD300. If I'm right the RCD510 also has the best rated amp out of the VW Headunits? Correct me if I'm wrong mind.. baring in mind if you want to do this you will also need an updated CAN Gateway which you can get from around £40 (cheap) hard to find at that price or £50 up over.
If you don't want to bother with head unit etc for now you could install the armrest which is nice in my opinion if you don't already have one. Not sure on price for these as i've never looked.
If you wanted to go further you could get some GTI seats for a bit more support when driving through the twisties. probably looking at around £300 - £650 ish for cloth tartan GTI seats. baring in mind theres also leather if thats your cup of tea and also the edition 30 half leather seats.
Multi function steering wheel if yours is just the standard wheel. MK5 or MK6 options available round the forum if your interested so keep an eye out. there are also other things needed when doing these like controllers etc so do some research on that as theres plenty of info on the forum.
For now thats all i can think of for interior wise besides the little seat inserts you can get which are nice little eye catcher when getting into the car. Around £10 i believe. And there is also the window buttons with chrome tipped edges along with the fuel flap release button / electric wing mirror button set etc which are nice. Not sure how much they are but you can find them all over the forum / bold sport website / ebay etc.
As for exterior, wheels of your choice.. no one can choose them for you really as its up to your taste..
Coilovers if you want to be low.. AP coilovers have a good reputation amongst the forum but it also depends wether you want to be on the floor or just lower than standard and have better drivability. AP for the second choice and something like FK's if you want to go ultra low for the 'scene' people. or theres always air ride if you have some poke to spend.
brakes - tyres - engine performance stuff - visibility (xenons) - handling mods like roll bars etc.. loads of threads, just spend plenty of time reading through some threads and use the 'search' function