So essentially I have three cameras which sort of work, but none of them are ideal, and I really don't want to fork out a hundred or more to get my Nikon or the Fuji modified to make them a better fit to infra-red work, or buy something ready-modified.
The Nikon still seems to be favourite for dramatic effects and art, the Fuji for ease of use; since I generally carry it every day I'll probably start taking out an infra-red filter too, it isn't exactly a big deal, but if I set out with the idea of shooting infa-red I'll take the Nikon instead. The Sony is actually the best for "pure" infra-red photography, but I don't like it much in other respects, so I'm not going to keep it.
Sorry that all of this is a bit inconclusive, but I still think that this is worth exploring because it can produce some very striking photos, especially with older digital cameras that have more IR sensitivity. All you really need to try it out and check your camera's performance is an IR remote control, if you can see the light from that the basic equipment is a 720nm filter, and that's not terribly expensive.