if you're planning on shooting 4x5("5x4" for you euro folk ), then I'd use a 10"(240mm) APO-Ronar, or RED DOT Artar. These were originally designed for flat-field copy work, and are apochromatic(all colors focusing on the same plane), imperative for digital work due to modern "flat" sensor designs, where as film is layered, long story...
These lenses are easy to come by, and there were even some that were as small as 4-4.5" long(100mm equivalent). Using one of these on a Nikon/Canon bellows(originally designed for macro use) with an adapter board will allow you to use it on your digital camera, if you go that route. They're sharp as all get-out, and also quite inexpensive on the used market. This way you can use the shutter in your camera.
A friend of a friend does this, with a similar setup as described above, only with a Hasselblad 555ELD, with a Leaf 33mp digital back. He uses strobes(Profoto) instead of a shutter speed(bulb, since its technically a "barrel" lens), and uses multiple "pops" to get the desired light output. Very simple, and very effective.
He shoots tethered to his laptop, and usually can get a painting(up to 30x40") photographed, and his setup set up, and torn down, in less than 2hrs. He gets $300 for copy work, and can hand off a RAW file to his client(who usually have more than one painting to be copied anyhow) at the end of the job. Very simple setup(polarized strobes in silver-lined softboxes(more specularity vs a white-lined softbox), like the above link.
Try your setup with shooting a flat couch cushion or something like a blanket(slightly textured) to test your lighting setup before venturing out on this. Better to know something than show up with no knowledge at all. Just sayin...
best of luck!
EDIT: remember that BRUSH STROKES add character(and reality) to copy work. They need to be sharp. It helps make a painting seem more "real".