I think there are two problems with this (even if the legal stuff is ironed out). First, over a long relationship, the expectation is that you have an equal relationship in terms of decision making, viewing each other as partners, etc. Its very difficult to do this if you have access to substantially different assets. You say he's letting you choose where you live, being generous, etc etc which is great and bodes well for the future. But - what if in 15 years he has a job offer, and you don't want to move, and he is the one who has the ability to sell the house and buy a new one? What if there's a subtle shift in power and in the families attitudes over time?
At the start, before you're married, when you've been together a few years, it all makes sense. As you consolidate your lives, it makes less so. This is why so often people talk about marriage of short duration. You can say now, well its his families' assets and I have no claim over them. And that's fine. But what if 30 years down the line you've been the lynchpin of that family, you've helped steward those assets, you've fed into decisions that have seen them accumulate more and more value and still they can throw you out on the street if he dies/you divorce?
The second big issue is there are just things that are unpredictable. I know two fit, healthy women in their late thirties who were the higher earners in their relationships who have both developed chronic and debilitating illnesses in the last five years. Neither can work now and that's unlikely to change, both actually are parents (though have still required childcare as they're not well enough to care for their children solo). One had an excellent illness-in-service benefit but after paying for five years she was having to fight the insurers to such an extent she rolled over and accepted it being stopped - they claimed she was fit enough for a staged return to work and no matter what consultants etc said they wouldn't listen so she gave up. So, they're entirely and unexpectedly reliant on their other halves, despite keeping their careers up after children and even paying for insurance, and relationships that were entirely equal in terms of childcare, who is the breadwinner, have changed substantially.
Its put pressure on both their relationships. If this happened to you, and you hadn't finished paying off your mortgage/nest egg, and the relationship broke down - where would it leave you?
In reality, despite the fact you won't have to contribute to mortgage/bills you'll quite likely be worse off in this relationship financially than if you married a man with less money who shared it with you. Because generally, you'd be buying a house with two incomes, you'd both be building the equity, and it would be a better house, and accumulate more equity (this is not 100% guaranteed but tends to be the case) so if you split and everything was 50/50 there would be more to split than you'll get from putting your savings into a buy to let. These are all things to think about.
Personally, obviously see a solicitor, but I'd also consider building in times of review in whatever is agreed. Personally, I don't see inherited wealth as 'earned.' Its v likely I'll have a fairly substantial inheritance at some point in the future - purely because I'm an only child and my parents brought property in an expensive area at the right time. If I get that inheritance, then DH and I split and he gets half....well, so be it. I don't really see it as 'my' money, I didn't work for it. Our relationship is a partnership, and our good fortune and bad fortune is shared. That is something I'd be worried about losing out on in the scenario you're entering into.