If you go with an older model, I would *highly* recommend budgeting for an Acute Matte D screen too, as they are incredibly bright and clear. That screen (along with the gliding mirror) is one of the major reasons I went with a 501CM. The viewfinder in my Pentax 6x7 MLU is very dim in comparison, and is quite difficult to precisely focus with using slower lenses and/or in dark conditions. Not so with the Acute Matte D. Mine also has a central rangefinder patch, which is a real boon for achieving critical focus.
Secondly, although it makes the whole setup heavier and bulkier I would also recommend a prism of some kind, as it really helps with critical composition & focus, and also protects the focusing screen (I personally never got on very well with the WLF). A 45 degree prism is most useful hand-held; I went with the unmetered PM45 as it is far cheaper than the metered PME45 (and I use a Sekonic L-758 DR to meter anyway). A 90 degree prism is generally better on a tripod.