Well I'm a budget traveller but have a look at this place.
on The Digital Orchidee Guesthouse
I stayed there about 3 years ago for the princely sum of $10/night A friend I was with paid $15 for some reason, perhaps it's my charm or it could have been the fact she had a room on a lower floor. There are more expensive rooms with direct access to the pool if you prefer. The room I had was nicer than the one with the loud wallpaper on that website...I guess they are all decorated differently!
For the price it was excellent value...very well maintained rooms (and decent plumbing), a brand new swimming pool, not far from some decent restaurants and a few minutes stroll from Ochheuteal Beach. You can walk to quieter stretches of beach with hardly a soul on them. Not 5* luxury but it had the character you seem to want without being a crumbling ruin.
Even though it wasn't very well known (not in LP at the time I don't think though it might be now) it was quite popular with people "in the know" for obvious reasons. The owner of the hotel I stayed in in Phnom Penh recommended it to me and said she stayed there herself. So I'd advise booking in advance. And although it was fairly full it was quiet there.
Just a suggestion anyway.