Hi Historic, I'd love to have some 'retirement time' on my hands, but I'm only 35 (ish

) so I'll have to leave that up to my dad who has just ticked over into 'retirement' mode.
One thing that i have tried to do with my '65's redevelopment is to try to incorporate as many late model items into my car as possible. You know, things that you can depend on being available (hopefully cheaply) at you local REPCO or Supercheap Auto (OZ references - Google will explain if required

).
It really depends on what market you want to cater for. If my father was running a Mustang business it would be 'factory original' all the way concern. If I were running a Mustang business it would be a "how I can incorporate as much modern technology into a classic shell' business. - stick to what you know best and what your 'restoration beliefs' will allow.
So, what are your strengths? Fabrication? Moulds (f/glass)? Engine building? Suspension tuning? What are your manufacturing capabilities? machining? cnc?
Well engineered bolt-on items are ideal for the 'joe average' backyard mechanics, but most of this type of items have already been done to death so unless you have something unique or better engineered you'll have trouble getting a foothold in this market.
Good luck with whatever you choose. :cheers