Personally I'd recommend using tractor or plant enamel - it's not (how shall I put this?) quite as sensitive to the preparation and application as car paints, sticks very well, is ideal for brush / roller application, covers a multitude of sins and is available in a wide range of colours at reasonable prices (check out eBay, your local agricultural suppliers or Google "tractor restoration" / "tractor paint"). I know that this will have some people spitting feathers , but with tractor enamel you can even get away without rubbing down on a reasonably well weathered Landy, just wiping over to make sure there's no oil or grease!
Most important thing is to make sure that there's a good key (rub down with wet and dry paper if the paint is all shiny) and clean grease-free surface (wipe over with white spirit / thinners or meths as available). There's no need to create a perfectly sanded base as per more traditional car paints.
I did my last Series IIA in dark blue with a yellow roof (approximately coastguard colours) and she was still looking very presentable when I sold her three years later - that was without sanding down at all, but the original paint was well weathered.