I think I saw someone else mention this earlier in the thread, but couldn't find it. At these magnifications you'll want to use a focusing rail. One driven by a rack and pinion or a threaded rod (like the Bogen 3419 Micro Positioning Plate) is best.
You can't focus effectively with bellows or by shifting the tripod at the magnifications you're talking about, so you need to move the whole rig (camera, lens, bellows) back and forth in very small increments to focus. It can be done with a sliding rail, but a rack & pinion or screw drive is much better.
Sorry to add to your equipment list. Have you considered reversing rings for stacking a couple of your existing lenses? See Shaw for how to do this and to calculate reproduction ratios.
BTW, I've seen good deals on bellows and tubes at the "camera shows" where people swap and dealers sell used equipment. I picked up a bellows with 39mm Leica threads on each end and a chimney finder/focuser (to provide SLR type viewing with rangefinders) a few weeks ago. It came with a 135mm f:4.5 Culminar lens and standard helical focusing lens barrel for $75. The results from the lens knocked me out when I saw them. (This is probably a late '40s early '50s bellows rig, perhaps Steinheil or early Novoflex.)
My son picked up a mint set of extension tubes for his Minoltas for about $20 at an earlier camera show.