First, assume that copyright/contracts/permission/etc taken care of for the gallery & artist, so all legal and above board.
The actual question:
So, I'm going to be taking some photographs of some artwork, primarily paintings/2d work for now, for the purpose of creating digital prints of the artwork. I've got a 5d2/5d3 (planning on using 5d3), pretty good quality Benro tripod & ballhead, shutter release, and know about mirror lockup & such. I'll have to get a good level, and then either eyeball it (probably bad idea) or bring a few things to try and cobble together something so that I'll be able to make sure the camera is square on to the artwork, and the plane of the sensor is properly at the plane of the art. I do have a couple of speedlites I was going to use on stands with either umbrellas or large foam-core boards at 45-degree angles on the sides as seems to be the general recommendation.
So, what lens? From my list, I was thinking the 40mm f/2.8 pancake as it's quite sharp and won't suffer much from perspective distortion. Plus I can keep it fairly wide open, f/4 or f/5.6 and have a very sharp image. Alternatively I could use the 85 1.8 @f/4 or so, but that feels a bit long for most things, and I'd have to have quite a bit of room.
I'm happy to rent a lens for a few days, but which one? The 24mm TS-E is too wide I feel, and would end up with some perspective distortion at the edges. 50 1.2 would be desirable over the 1.4 for renting, but I don't know well enough about it's performance and 40mm vs 50mm? I dunno how different in perspective it is if it'd be worth it. One of the Zeiss 50mm? Canon 45mm TS-E? 90mm TS-E? Canon 50mm f/2.5 Macro?