I think we may have a problem with the definition of "prime." In the U.S. at retail only three grades of beef may be sold: prime, choice, and select. Technically, these are the three grades that food service operators of all genres should be using. Choice is most often used by caterers, and I would venture a guess that 75% of prime rib served is not prime, but choice. Choice is significantly cheaper than prime, particularly dry aged prime. If your meat supplier is selling USDA prime graded whole rib at $5.75 lb., he's infinitely below market price in most major cities.
For those curious, beef still has multiple grades: prime, choice and select, as already noted, but also standard, commercial, utility, cutter, and canner. Select used be graded USDA "good" but grocers lobbied to have the grading name changed because it sounded better in their ads.
Regarding quantity, I'm not certain that 100# will be enough. A cooked and trimmed serving of prime rib is traditionally 6 to 8 oz. when served at events such as the one being described. Even if these rib roasts are roasted low and slow, you can plan for a minimum reduction of 20% shrinkage and trimming. You may get 80 lbs. ready-to-serve. If you can hold the portions to 6 oz, you'll get 200 servings, as shroomgirl noted. If your carvers get generous and cut 8 oz servings, 160 servings.
Another problem is the ribs may arrive overly fat and you lose another 5% in the roast and trim.
Two posters suggested prime rib in their posts.
And, yes...Graded choice boneless rib roasts, preferably rolled and tied from your supplier, about 110 lbs. are an excellent choice.
An aside: Check with all your local beef suppliers, including a member of said church, who may be a grocer. In some regions you can purchase whole beef tenderloins, silver skin, however, intact for under $6.00 per pound.