As a general rule, a modified car is more difficult to sell than a stock car. The reason for that is that there are many people who will only consider a stock car and will not even look at a modified people. The more significant the mods, the more the number of potential buyers is reduced. However, that doesn't mean that a modified car is unsellable; it simply means that you may need to be a bit luckier in finding someone who is willing to buy a modded car or, even better, one who wants one with exactly the mods you have made. Or, it may mean that you have to sell the car for less than you would if it didn't have those mods.
Regarding the market value of a modded car, there are many, many mods that will not raise the market value of the car at all. Spend $1,000 on a mod and the market value doesn't go up (and could even go down due to the smaller number of buyers). There are some mods that WILL increase the market value of the car. I consider any of the major supercharger kits in this category. An NSX with a Comptech or BaschBoost or GruppeM supercharger is worth more than one without one. However, it is not on a dollar-for-dollar basis. Spend $10K on a Comptech supercharger and it may increase the car's market value by $5K. (You can remove it and sell it separately, but again, you won't get the original purchase price back.)
My recommendation is this. Get the mods if they're worth it if you weren't going to make or lose anything on them when it comes time to sell. For example, if an aftermarket exhaust costs $1500, get it if the enjoyment of the exhaust while you own the car is worth $1500 to you. If you're worried about how much money you're going to get back from your "investment" in a mod, you probably shouldn't get it. If you don't care about how much you'll ever get back and you still want the mod, then get it.
I also would strongly recommend not committing to any mods until you drive the car for the first month or two. After doing so, you may find that you don't need to modify it at all - or that you have different priorities for which mods to make after owning it than you were guessing at beforehand.
[This message has been edited by nsxtasy (edited 22 May 2002).]