seeing as you asked mate il spurt out a list of mods i did to cars at your age,,,,not forgetting about insurance,if you do or you dont declare mods is entirely up to you but remember as soon as you change the appearance of the car your insurance could be void,,,in some cases this can include wacking a few stickers on your car,something to keep in mind,,,i was a bugger back when mcdonalds car parks was the place to be but ive learnt the hard way
haynes manual was the first thing i got for a new car,,,,you can remove/fit 99% of the car and it shows fault finding,maintenance and correct torque settings when refitting parts
full service including plugs,leads,filters,grease,oil everything mechanical,,,some cars loose a few bhp over time and if wanting to fit an airfilter and exhaust you want to be starting with a good engine
new discs,pads and check the brake lines,,,getting your brakes sorted or upgraded isnt just so you can stop quicker,it makes your car alot better to drive out on the twisty country roads and you feel alot safer as you dont get so many a55 in mouth moments,,,,,,,,,but it will happen,trust me,,brakes are good,keep them right
get your suspension and wheels setup,aligned,balanced and some decent tyres,maybe even a set of standard alloys,,,,its nothing like lowering a car 60-100mm but it makes a difference and gives abit more grip,again feels alot better when out on the twistys
you seem to be off to a good start with detailing/valeting,the list is endless what you could do to make your car look brand new in a weekend,,,clay bar,buffer,few touch ups with a few subtle mods,adds value if you come to sell
you could do all the above for a few 100 and have a mint car without affecting insurance,then when you come to fit a turbo,rollcage,coilovers or an ice install to pi55 the neighbours off you would have a perfect base for tuning,,,,,,,,,,all those times playing drum and bass in the driveway when fitting subs,amps and flashing lights,,ahh the days