Although Kylius Lonaro doesn't respawn, his marauder henchman does. That means you could have to re-kill him every three days, and of course, he'll get harder as you level up. Making it a viable player house would require the spawn point to be disabled (not just the current marauder spawned from it, so not a console job unless you get the right formid from the CS).
Making a mod that does that wouldn't be hard to do, You'd add a quest (e.g. Greylands) with a script that's essentially
scn GreylandsScriptbegin gamemodeif GetStage SQ02 < 100 ; did the other quest end normally? return ; do nothing until SQ02 is completedendifif GetStage SQ02 == 200 ; did SQ02 fail because Lerexus got killed? StopQuest Greylands ; or whatever you called itendif; do whatever you need here, including disabling the spawn of the marauder, StopQuest Greylands ; only need to do this once, and we're doneend
My suggestion would be to replace the door with one that leads to an alternate interior (created by copying the interior cell) furnished the way you like. The marauder spawn point is then left untouched in the original interior cell that can't be reached any more, so you don't have to worry about it.
Both of those quest stages are advanced while the player is outside Greylands (either because you're back at Lerexus getting the reward, or witnessing him getting killed) so the interior swap won't be seen happening.
This would make a good "first mod" project. We'll soon have another CS addict!