As a general rule, I would go with a heavy duty roping saddle for ranch work. A "ranch" saddle is just a durable, all around, heavy duty cutting or roping saddle - probably limited embellishments. It's a work saddle.
As far as tree size, I'd go with full quarter horse bars. If you get a chance to speak with the saddler, they can discuss with you a midrange gullet width - not super wide, but not crazy narrow either.
A working ranch horse will usually be quite stocky and broad. Of course, there are exceptions, but these are horses who must be able to brace against the force of a running cow. Ropers are big and broad. Cutters are shorter, but probably just as broad.
You will likely encounter horses with a gullet width of anywhere from 6 - 7 inches. I'd go with perhaps something along the lines of 6.75, or 7. However, saddle fit is a key factor in how a horse performs - so try the saddle on the horse(s) you'll be riding before you start working, and if it doesn't fit, ask to use a saddle that fits that particular horse better than yours.
You will also need a heavy duty working pad. But please, don't skimp in this area. A low quality pad can be just as bad as an ill-fitting saddle in many cases.
You will probably need a leather flank girth, leather breast collar, and leather headstall, and a high quality cinch. You might consider getting a set of SMBs in medium if you think the work will be strenuous on the horses' legs.
Best of luck. :]