What I have had done myself is:
ITG Maxogen £200 (second hand off a mate on RS, they are aboout £350 new) easy to fit so no cost there, reported gains from ITG are 7bhp increase
Wheels £635 from performance alloys and Tyres £235 from Camskill, fitted at a local garage for £10 each
De-chaved various bits on the car (rear lights, replaced with standard ones and front grilles replaced with standard ones at very small costs as I bought them in the for sale section on here)
Exhaust was already fitted when I bought the car, had a few little problems with it as it is not the best quality but I like the sound of it, other good systems to go for are yozza, janspeed, orbisound or ktec, worth taking a look into those if you are thinking of changing that, will cost you around £500
Remap you can get done by Paul at RS Tuning will cost you around £200, very worthwhile doing (but only after mods such as an air filter or exhaust, not before you put them on, will see an increase of up to 15bhp from that)
Next on my list is to upgrade the front brake pads to brembo max discs and ferodo ds2500 pads, a combination that you will find on most 182's as its a very good upgrade and will cost about £200
You can probably find yourself a splitter in the sale section on here for a cheap price, but they do get snapped up quickly, another one is a carbon fibre front splitter but thats costs £100 and is actually heavier than a standard splitter lol
Suspension is going to be a killer as unfortunatley the previous owner put shitty apex springs on it, so I am going to go for some coilovers which will be about £900, not a cheap mod lol but very worth it!