<p>When I first bought an MP-E 65mm, I expected that I would mostly use it in natural light, with a focusing rail. However, in practice, I rarely use it that way. I use it almost exclusively with flash. Like William, I dislike the look that ring lights give, so I prefer to use the Canon macro twin light (MT-24EX). I find that I can use it hand-held up to about 3x magnification. The subjects are mostly tiny insects that I'd like to see more clearly. It can be hard to hold the camera at the right distance from the subject to get well-focused shots but I find that if I take enough shots there are nearly always some sharp ones.<br>
If you were to take this approach, I'd recommend a used 5D because it has a better viewfinder than APS-C cameras. The original 5D would be fine, and it's quite a bargain at the moment if you can find a lightly-used one. If you prefer the focusing rail approach, I'd recommend any camera with live view, because you can magnify the image on the rear screen to be sure that you've achieved critical focus.<br>
Have you considered renting the lens and an EOS body to see if you like it?<br>
As an aside, someone suggested putting insects in the freezer to make them easier to control. I can't condone any chilling of insects in order to photograph them. However, if anyone feels that they really must do this, the fridge would be a more considerate choice than the freezer.</p>