Improving safety, reliability, and power will not hurt resale - they'll actually improve it. "Improving" the appearance might be a matter of taste, but tasteful easy to undo changes should actually improve resale. A quality paint job also improves resale - just take some pictures so you can show the buyer what's underneath - I'm always leery of a freshly painted car with no photo-documentation on what it looked like before and when stripped.
When you do sell, just send the original parts along with the car so the new owner can go back to totally stock if desired.
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