I'm pretty sure that any car bought today won't have any collectors value in the future. Exceptions may be Vipers or the ZR-1/Z06 Corvettes. Any Mustang 2005+ can be "converted" into a Shelby...by Shelby...if you ask them to do it. It will be listed in the Shelby registry, too. Therefore, any Shelby Mustang today is worth nothing tomorrow.
That said, modify as you like!
As far as the 401k...LOL...umm...yeah. Good luck with that one too. The dollar will be long gone by the time most of us are to retire.