NightWind
possible house for him is a mage tower since that seems to be closest to his own interests and he has a high budget to work with so it works he could even go overboard on the interior to allow some truly magical rooms more so then the standard it would also scream that he is a powerful and highly skilled mage both the area it is in the exterior and the interior plus the numerous crafting and research rooms and likely massive library(after all last i checked spell books are physical books so they would work here and he rich enough to buy a large amount of them) would help
i can think of a way to get around the rules that say you cant harm those in a safe zone catch though is you need to be a spatial mage to do so the method takes inspiration from portal games simple make a portal linked to a area outside a safe zone and toss the individual though the portal they are no longer protected such a spell would be extremely rare due in large part to how rare spatial mages are making it the sort of thing no one prepares for
one thing i know about both fantasy and scifi setting is there tends to be ways to increase lifespan or decrease ageing its pretty common thing in such setting in that it appears in most but in such setting it tends to be either very rare or very expensive and would likely make a good reward it wouldn't be out of place given the nature of the tower and what it can do there are already extremely long lived races in the tower so there should be ways to make it work