If you don't already have the Nikon, consider buying a Sigma DSLR - either the SD-14 or SD-15. They have an IR cut filter that doubles as a dust filter, that is really easily removed and put back in.
With the cut filter removed the camera is fully IR, you can then add something like a Wratten 89b filter to the lens to get the traditional IR look, or shoot a kind of "full color" IR.
You can probably get one of those cameras for less than the cost of conversion alone.
The earlier Sigma DSLR's also did IR, but the resolution is not as high and the dust filter required a screw to be undone to be removed, instead of being able to remove by hand.