As the others have said You could spend a fortune for very little gain.......
Induction kits
Exhaust
Cam's
Ecu Trickery / Remap,
Lighter wheels,
Stripped interior,
upgraded this that and the other...
BUT Then you should / Must Tell your insurance company what you have done... They see this as a green light to mark you down as a Boy racer and charge you as a higher risk..
Then when you come to sell it you will get naff all for it as any garage will not be able to sell it.... As most car buyers will see the fact its been "played" with and not want it as in their eyes it will have been ragged about all over....
That and certain bits will be more likely to fail prematurely giving you an unreliable car.....
By all means do it but just expect to throw good money after bad where you could probably part ex it buy a faster car and insure it for several years with what you will save by leaving it standard and selling it.........