A 450D would be perfect for a conversion: it already has live view and live view AF. Paired with a 10-22 EFS (with the AF system calibrated for it) it would be perfect, as that lens has no hotspot in IR (don't know how sharp canon lenses in IR are though, in Nikon the equally hotspot-free 10-24 and 16-85VR lenses, while being about equally sharp in visible light, the 10-24 is softer in IR than the other.).
Also, more recent bodies (500D, 550D, etc) have higher MP count (higher pixel density) which is a bad thing as diffraction kicks in at f/8 (on a 12MP sensor) in IR already (in Nikon at least, in Canon it should be even worse and the Canon APS-C sensor is even smaller than the Nikon DX).
The only problem with the choice of the 10-22 EFS for IR would be the focus shift: it has a huge amount of focus shift in IR, if you want it to focus correctly, you probably won't have another lens that focuses near to it, not even with live view.
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