Nerb, it's a bit like asking how long is a piece of string.
I'd start with an overhead crane, a lathe or two, a milling machine, a wet saw, pipe benders, a tig, mig & stick welder, 10 tonne press, forge, anvil & anything else I could possibly buy if I won the lotto.
Seriously though, everybody dies of old age before they actually ever finish their workshop just the way they want it. The older people get, the more they learn to accept that every shed they ever build will be completely full of junk and clutter before it's finished. The answer is simple though, just keep building more sheds. Life is incomplete without them.
Cheers, Willie.