its simple really...if you like the car and want to keep it, and are doing the mods to make it more appealing / modern / faster etc for YOU, then go right ahead....i am about to spend about $8k on an escort! it will increase your enjoyment of your car for you.
however, if you have any plans to sell the car in the near future...then dont do a thing. you will lose at least 50% of what you spend on it.
You wash your car like it was your firstborn child, you tend to its needs like it was your own body, you protect it like it's your family, then you drive it like you stole it.
Weapons of rice destruction
2002 Ford Falcon AU3 XR6
1985 Ford Sierra XR4i
1980 Ford Escort RS2000