Jon Ess , Aug 26, 2010; 01:30 p.m.
I would go with a camera that has live view; with that feature you can see what the sensor sees and focus appropriately. Infrared wavelengths focus differently than visible, so without live view your autofocus will typically be off. This can be compensated for when the conversion is done, but different lenses will still behave differently...with live view you will be able to get accurate focus with any lens.
I think the first Rebel with live view was the 450d/xsi...so I would go for one of those or a newer model.