I am not going to let a single outrageous sale of an Eleanor clone dictate the entire market. That being said, 99.999% of cars are a crappy 'investment.' Your car, do what you want. Suspension improvements, electronic ignition for reliability and a transmission with overdrive just make for a nicer ride. If your ride is in really good shape and incredibly original, I would think twice about even those mods as a car is only original once and I personally feel that if I find something in excellent undisturbed condition then I am obligated to be a steward of that. YMMV. If you are wanting to radically modify your ride here are a few things I have come to realize:
1. The more 'unique' you make your car, the less audience. Sometimes to the point of only an audience of 'you.'
2. Hot rods are usually built to a common formula and Mustangs are no different. By sticking with common creature feature mods you maintain audience, but look like every other modified Stang out there.
3. Taste is relative, but conservative style is always relevant. i.e. less is more.
4. The biggest mistake is to bite off way more than you can chew. This is usually accomplished by not having a plan, under estimating your skill set, time and available funds. It usually ends with the attempted sale of a pile of parts that used to resemble a Mustang with some poorly executed body work. Remember, 'Restoration Started' really means 'I took it apart and lost interest.'
5. Drive it / change something / drive it /change something etc seems to work for most people, plus you get to actually enjoy the ride vs staring at the giant pile of disassembled 'potential' you wife wants out of the garage. (See 'Lost Interest')
6. Harness your enthusiasm and pace yourself. Restoration/modification is a marathon, not a sprint.
Good luck with your project. You will always be able to get encouragement here!
How about some pics?
BTW: My convertible is in the middle of some radical mods. It came into my life as a basket case C code. My wife's coupe is not. It came into our lives as an all original in need of some care. So I'm on both sides of this fence and quite frankly, I do not see any conflict in being so.