I think people have tried before it’s usually flawed in that not a huge number of people want to rent them, and they’re put off by large deposits required to ensure such expensive bikes are simply stolen. But there are lots trail centres (usually the larger ones) that rent bikes out, because they have a market for it.
Although shops like torico near UKBP, pedalabikeaway near FOD etc. will normally let you use a demo dh bike for a day if you ask and are interested in buying.
Or just get up to the dh track near you on whatever you’ve got currently, meet people, make friends, try the tracks and try their bikes.
You can rent scott gamblers and other dh bikes from ps cycles near cwmdown. They have a big range of downhill rental bikes at fort bill too, so you could make a weekend of that if you wanted. Go with some mates, pitch a tent and have the full dh experience without buying a bike until you know you want to. Then there is glencoe and the nevis range who also rent dh bikes.
Also – ring the trail centres. If your nice there may well be people at the centre who will lend you a bike for a deposit if you pay for a full days track access, but it depends who you speak to.
Edit: sorry I thought you were posting from a personal POV not as a possible business opportunity. I would advise you to think very carefully before buying a fleet of dh bikes for rental – there are hundreds of pitfalls and there many very good trail centres out there who avoid rental for those reasons.