In addition to the more recent versions, you can also use the older "N" (designed for the Super) metered and un-metered prisms with the Pro. The FK402 finder will also work as well.
Personally I prefer the left-hand grips with the proper (RA402) connector to the right-hand grips/power winders, but YMMV.
There is no such thing as a 120 or 220 back. To switch between the two, you just switch inserts. If you find a good deal on a back with a 220 insert, just buy it and replace the insert with a 120 insert.
The 150mm LS lens should be relatively easy to find. You may have more trouble finding the connecting cable that permits auto-cocking with the WG401 grip.
I do like having the meter in the prism, even if I am using a hand meter mostly. If you intend to purchase extension tubes or other macro accessories, the built in meter is very handy.
In case you don't have it, here is a pdf of the Pro-Tl system chart: