Any info on mods for my 205 1.6 gti

yo ive just got a g plate 205 1.6 gti and im only 15 and could do with some info on modifications on the car the main mods i want to make on the car are 1.9 alloys, leather gti interior k & n filters and plus my gearbox synchrones are worn so any info would be appreciated and for alloys do they have to be 4 stud thanx