OP, it all depends on budget.
Re lenses, if you shop hard you should be able to find a 58/5.6 Konica Hexanon or Omegon as fitted to Koni Omega/Rapid Omega shutters. They can't be used on other cameras, have to be reshuttered in a normal #0. The 58/5.6's advantage of the older 60/5.6 is that it can be mounted in a #0 press shutter. The 60 has to go in a cock-and-shoot #0. Both cover 2x3.
Re roll holder, most of the ones mentioned attach to 2x3 Graflok backs. The exception is the Mamiya Press holder, which will attach to a Mamiya M Frame. I've been quite happy with my Adapt-A-Roll 620 roll holders, which slip into a spring (Graflok too) back like a sheet film holder. They will feed from a 120 spool but must take up on a 620.
Since no one has mentioned them, the Linhof Super Rollex is one of the best. Ed, the Techno Rollex is a 6x12 roll holder, the Super Rollex is (was?) made in sizes to fit 2x3 and 4x5 Graflok backs and a variety of gate sizes including 6x7 and 6x9.
OP, since you're building you shouldn't rule out the possibility of making a camera that will accept a roll holder made to fit a 4x5 camera. Then, if you have the money, you can get a 6x9 Sinar Panorama.