So what bike should you choose?
Something that appeals to you, possibly even something obscure or rare - it's your choice but you have to like it because you'll be spending a lot of time with it. If you can't bear to look at it, you're never going to put the effort in.
Think about what you want to do with it when it's finished. Ride it? Sell it? Look at it?
If you plan to take on that Europe trip you always dreamed about, choose something that is big enough to cope. That BSA Bantam will struggle two-up, with luggage, popping to town let alone thousands of miles through country after country.
If you want to sell it afterwards, look at the prices they go for now and do the maths. Bear in mind you could easily spend a few grand bringing an old bike back to life. If you have to do a full restoration, getting back the money you spend is harder than you think.
If you just want to have a pristine example of something - it doesn't matter what it is. Just enjoy it.
This AJS Model 30 pretty much got back the money it cost to restore including original cost. That's doing as much of the work as possible, ourselves. So think about that. A couple of years spent restoring a bike and just about broke even when selling.
The hours spent can't be counted - it's a hobby and a passion. Nothing else matters.