It's also going to make a difference depending what camera you're going to use.
Most 45's, but not all will compress the bellows enough to use a 90, but shorter will definitely need a bag bellows or recessed lensboard. Some cameras that allow compressing the bellows enough may not allow movements. You would need interchangeable bellows.
Reread all this. Everything here is spot on and great advice for someone starting out, especially the last line! :)
Originally Posted by Dan Fromm
I would add one thing. Caltar lenses can be a bargain sometimes. The later ones are rebadged Rodenstocks and Schneiders. Same lens, cheaper price.