"Tasteful" is a very subjective term. The phrase "to each their own" comes to mind.

About the best definition I can offer would be to simply not go over the top...

As for mods adding value.. In general, modifications add none and to the general public they actually reduce a cars value. With that said, from time to time the right buyer is out there that will appreciate the changes and you may can get a premium price for it, but a bank will rarely "vlaue" it any higher than one without so even if you find a buyer that "likes" it, you often will still find yourself selling it for the same as someone who has done nothing.

If you worry about value, hold onto the old parts. If you go to sell it, try to find that RIGHT buyer who appreciates your completed works. If one doesnt show up, you can always put the stock parts back on, sell it as stock and then sell the parts seperately.