GGR,
You have proposed a long, comprehensive list of changes that will take a long time and a lot of money. Just on that note alone, don't expect to recoup your investment in them. That being said, here is my take on some of them.
1) Wheels, Sway Bar, should not have a negative impact on much of anything as they can easily be reversed back to stock.
2) Bushings are hidden, and if they improve your ride they too will probably be OK and relative easy to revert back to stock if you or another owner does not like them.
3) Deleting the A/C is pretty common when people can't get it working right. However, a well-sorted A/C unit is wonderful to have if you drive in the summer. Even top down in a D.C. July is miserable. Want to delete 60#? Keep passengers out of the car; drive with 1/4 tank of gas, leave the spare tire and tool kit at home.
4) Transmission is an often discussed topic here. If you change anything ensure it is done properly, no quirky half-baked conversions, Mickey-Mouse mechanical work, or things like that. Consider a rear end conversion if the RPM's bother you. Another take is that it isn't broken, so why bother?
5) Do things for a reason. Why does the battery need to be relocated? If you want to improve the battery from stock, get an Optima. It does not need to move.
6) Stainless exhaust will be the last one you buy...unless you don't like the sound. Some don't like the "note".
7) Euro look conversion is popular. Many have done it.
8.. Electronic ignitions of any type are generally an improvement over stock, but better make sure all else is working fine first.
You state that you want to make a "nimble and fun car" to drive. My car, though hardly original (highly restored) is mostly stock (meaning original design parts, OEM, etc.) with mods such as electronic ignition, stainless exhaust, seat belts. I find it fun and nimble to drive as it is--as most Pagodas are. It does not have a/c, and it is an automatic. Yes, it's screaming at 4,000 RPM at my highway cruising at 70 MPH...just as Rudi Uhlenhaut intended. The USA spec does not bother me--it was built that way. (Besides, when I'm driving it, OTHER people see the side markers, and headlights and most don't have a clue either way...
) I don't often think about battery location. I have the aluminum wheels, but keep the steel ones around.
Putting dollar value effects on such a list of mods is highly subjective. Originality is something a lot of buyers look for--never heard of anyone seeking out a modified car.
John Olsen, from the SL Market Letter, keeps a large database of SL and collectible sale information. Reading his condition chart, there is a difference between modified and original, particularly if the mods are done "carelessly" or "amateur". Subjective but deviations from OEM or stock devlaue the car--period. That's the data. However, there might be some special allowances. Peter Lesler here, has created a "competition" Pagoda for racing time trial and autocross. That requires some serious mods. If someone is looking for a pre-made car of that kind--well, perhaps the mods were proper.
Bottom line is if you are going to spend all this money, best to keep the car, and enjoy it. Don't turn around and try to sell it when complete--you would probably be better off leaving it alone and just making what you have work well.
Tom Sargeant (DC area and this site) has made a lot of mods to his car for drivability and they were done nicely and work well.