The Roberts 58 is a family of designs, with some big variations in materials and deck layout. You have not said how finished this boat is and which configuration. In its most sailing oriented version, I think that this is one of Roberts better designs, but the full blown sailing version has something like 7 feet of draft and 20,000 lbs or so of ballast. If the boat was intentionally only finished as a powerboat, there is a good chance that the ballast keel and all of its internal structure is missing. If built as a power boat, or even the motor sailor version, it would have a smaller rudder and the full support structure for the rudder might also be absent. Similarly, all of the chainplates and internal support framing for the chainplates might also be missing, as would the deck hardware and its structure as well.
While these are not the best sailing boats out there, my sense is that they are reasonable sailors if built as a sailboat with a full depth and fully ballasted keel. 58 footers are big boats, too big for half-assed hardware and rigs. This is an over 60,000 lb boat. When something goes wrong on boats that big, people get badly hurt. Rigging a boat this size no joke. Untempered aluminum light poles and galvanized steel rigging won't save enough to make the risk worth taking.
For the most parts Junk rigs work best on smaller boats, and boats that are specifically designed for a junk rig and a junk rig on a boat this size won't save much money either. A freestanding rig this big will add a lot of cost with zero advantage on a boat this heavy. While the masts for ajunk rig may be a little cheaper, and there is no standing rigging to speak of, there is vastly more sail cloth, vastly more labor to build a standing lug (the actual rig type employed on so-called junk rigs) and vastly more running rigging, several extra spars, and vastly more blocks, and perhaps a few less winches than would be found on the rig the boat was designed to have. Offsetting any savings in the mast and rigging, is the sheer amount of expense involved in building the internal structure to take the extremely high loads at the deck and keel that is exerted by a freestanding spar. The load at the deck is literally several times the overall loading on the sail and rigging, and that load must be absorbed through the cabin structure which is normally the waekest structure on a boat this size. Not a very good idea at all.
Wood spars are fine as long as you use quality materials so that you do not get too much weight aloft, but the last time that I ran a cost comparason between a properly built wooden spar and an aluminum spar, the aluminum was a real bargain.
Then there is the whole winches thing. I know you can get by with buying big hand cranked winches but frankly, most boats this size rely on electrically powered winches or else coffee grinders. It is pretty unlikely that you will find used winches for a vessel this size.
I think that who ever estimated $100,000 is probably the closest to the real cost to rig a boat like this, if you do all of the labor involved, including welding your own fittings and doing your own paint and rigging work, but my deep sense is that would be a real bargain if you could do it for that.
When I worked for a yacht designer and when my Mother was importning yachts from taiwan, we generally figured that the rigging and sails were roughly 15% to 25% of the cost of building a sailboat. If you figure that a newish 58 footer is one million to 1.5 million, that would say that you should expect the rig to cost somewhere between $150,000 and $225,000 and that assumes that you have a ballast keel, and rigging attachment structure.
Respectfully,
Jeff