HARRY - I don't have any experience with the Sun-Master gang reel mowers behind CC's, but a company I used to work for used an old refurbished Ford tractor and a 7-gang reel mower to mow their HUGE 4-5 acre front lawn. Thing took a really wide swath, the old retiree who came to mow every couple days didn't mow fast but it did a beautiful job mowing, took two zero-turn riders flying over the yard to mow as fast as he did when he stopped mowing due to poor health.
The reel mowers are touchy to set up correctly to mow well, the blades are fussy to sharpen since they shear the grass against a stationary cutter bar but they're really good for the grass. And since they are only 21 inches wide they follow ground contours. They don't like twigs, sticks, or other debris in the grass, dulls the rotating blades and shear bar, plus the sticks can cause the rotating reels to stop.
They pull really easy, a 149 on a 3-gang mower is WAY over-kill on HP. A 7 HP CC can easily pull a 5 gang mower. They don't mow tall grass or weeds well but they cover the ground so fast you can mow much more frequently. They work best on dry grass of course, so the drive wheels don't slip.
Your SON made a great find there. That will make a nice display at shows when it's all cleaned up nice. Be fun to mow with too.