Keysafes are brilliant. I believe different makes have different combination requirements, ranging from 4 to 10 digits, and I’d recommend getting one with more rather than fewer numbers in the combination. Yes, that’s a pain for users, but it stops the clever thief from trying to hack into it.
On siting, I’d recommend having it somewhere in plain view but where the opening operation can’t be overlooked - think of cash machines and PIN numbers. A casual thief’s not going to risk trying to get at the keysafe in plain view. Sad to say it, but the professional thief’s going to get in to any building anyway, with or without a key.
Of course, I know someone who doesn’t worry about whether he’s got his keys on him because he can always rely on there being a key in his keysafe for his companion of the day to fiddle about getting out.