Arthur, as Øivind suggested, UV, iR and UVIVF are very different areas of photography. So, it is essential to decide which kind of images you want to make in the first place. The necessary equipments are also very different.
If you just want to get the feel of IR photography, the cheapest way should be to get a body with the inefficient IR-cut filter (D70, D70s or D40. D2H is also good, but more expensive), because you don't need to modify, and an IR-pass filter (which cuts off at 720nm or denser). Some lenses you already have may work well enough. If none of your lenses would work, get an MF Nikkor 28/3.5 (either single- or multi-coated or K or Ai will do). You also need a tripod, but you should already have one.
UV photography is a totally different league. That said, D40 can be used for UV without modification, so it can serve you a very nice body to start that is very kind to your wallet.