Well i have modded serveral cars in the past, so a different more open reply from one that has done it. If you change wheels, suspension, filters, exhaust etc then you must tell the insurance company, most add a fee, some dont.
First things id do is make sure the car is clean, youd be suprised the difference a good t-cut can do. After that 17inch wheels look daft with big arch gaps, so should only be done after lowering, which is you do lower, dont waste money on just springs it does cock the ride up, properly mached springs and dampers are great, if you prefer a stiffer firmer rider, it will give you more feel etc but with a 1ltre Corsa you'l never really feel the benifit from them. Low profile wheels give more feeling and better response to the car, but again they add a bit more harshness to the ride, and can tramline, if large wheels too they will slow the cars acerlation but improve top end speed, which again wont help a 1 ltre. Big wheels on a lowered car, clean car can look nice though, but silly on a dented car with mutiple panel colours, or if they are toooo large for the car.
Body kits can look ace if done right, but means getting the kit, getting painted, and fitted at the same time to match the car, and done properly, but again looks pants with small hub cap wheels.
Filters and exhausts, a good filter sounds nice, but adds vitually nothing to normal driving, exhaust a good one sounds ace, but again adds vitually nothing. But on such a small engine you might not really even get the ace sound, it would deffo feel more like you reving the thing to death and not actually getting anywhere.
Sound systems you can get speaker upgrades for just about any standard speakers id get those every time fitted in the standard pods, i have doen that in every car i have ever had, standard or not, problem with older smaller cars though if you start fitting big subs etc the car rattle like .... i have the standard head unit in my car and is enough for me, my brother just got a new sony , an better speaker in the door, and when i heard it i thought he had fitted a sub in the boot. Sub's in boots canbe a pain in the arse, as they take up space i did mange to fit 2 small mid range cones under the front seats to massive effect.
Other engine mods can make the car very hard to drive on the road, and make it unreliable to.
I love car modding, i dicided early that one car was going to end up as a track car, so started making it clean fitted lower suspension and wheels at the same time as a rear spoiler, one off exhaust and filter. Looked nice and sounded ace, but was as slow and harsh to drive, horrible on motorways. But i still loved going the back rounds purely for the sound on full throttle which you could do alot. The only sound system on that was the throttle pedal, and cat1 alarm. Total cost including basic car so far £7Kish
Then i got a stand Ford Focus, that was going to be normal car, except i removed a resonator from the air intake to improve the induction sound. New car £9K, was faster cheaper to insure, and handled ok not quite as sharp to be honest, but was more comfortable.
Back to the other car, new uprated bushes roll bars, new steering rack, rebuilt the engine rebored forged internals custom ecu cams etc bigger brakes, gear box, fly wheels est another £8K total so £15K over 2 years, the car now is in a state where it sounds ace, goes well, but its still a complete nightmere on the road, and had to be recovered a couple of times to as it hated sitting in traffic, but the fun factor to be made it worth every penny when you could open her up, which was pretty much just on the track, with full on race adjusted suspension. To sell it i wouldnt even get £2K for the complete car.
Although id had though of that before hand my next toy car would run the same engine. Was planing on a Elise, but couldnt ever get the money, so settled for MGF (£2K), which i actually blow the engine up before i had a chance to fit the other engine. Nice fun car car having 230bhp but i wouldnt have been able to live with on a daily bases. I broke the other car for spares, and when i had a few problems due to a crash at work i had to get of the car, i made £3K back in total. Thats £14K lost in 2 and half years, but that dosnt include the insurance for just the modded cars of between £1K and £1.5K a year. I had specific insurance in that i listed every thing, so if it had been stolen you get back the value of the parts listed, but it cost a fortune compared to normal insurance.
Do i regret it no, as i had more fun on track then i ever thought was possible, although i found out i could have brought and raced a formula legend race car for a year for the amount i spent.
Car modding can be fun, if your hands on and do the work, or if you have more money the sense, as it becomes very expensive, but you never reclaim the money poored into it, but its nor really that much different to spending £700 a year on sweets for example..
A clean car will sell easier, but wont add a great deal to resale value to a car of your age. But only you can tell weather its worth the effort or compromise to your life style, based on you income at the time. But modded cars become a target not just from joy riders, but car breakers to. Your call.
Best bit of advise i can give though in general is not to get yourself in tooooo much debt doing it, as thats when it becomes hard to get of again. I got in debt for a none fault acident at work and the time i had to have of work, so again know that from experiance. My out look is i do what i enjoy doing whn i have the money to do it, never think i'l be losing money, just can afford it yes, will i enjoy it, yes, then lets do it, whos to say we a all going to live another 5-10-15-20-25 years. I rather die have enjoyed my self rather sitting there at 80 thinking to myself why didnt do X when i was X while had the money and life in me to do it.