When we moved, we found that the central vac in the new house had little/no suction and for whatever reason, DH did not investigate further. We bought a Dyson upright and I hate the thing with a passion. Only good thing about the experience was that we did get a good deal on the thing. It is horrible to move up and down stairs, is hard to maneuver and does a lousy job on hardwood. It would definitely chew up the fringe on your dad's rug.
Back to the non-functioning central vac--after I complained enough, DH decided to check the thing out. There was a broken connection in the tube in the basement. When he fixed that--a very minor repair--it works very very well. I especially like that it has outlets on every floor so it is just a case of moving the long hose from floor to floor.
My neighbour has a Shark and loves the brand so much that this is her second. She wore out her first.
Research is definitely key. But for me, having sold a 2 1/2 storey house with central vac and now having a 2 story house with a repaired central vac, I would definitely investigate the cost for installing such a system in your Dad's house. It would also be a selling feature down the road. And definitely look at the bag-less systems. The cost of something you are just throwing away, can really add up after a while.