Montly income

The big question is how fst you spend!
Also, how much gor the housing. ...as prices for a rental go from 150 $ to 1.500 $ it's hard to say.
You wil certainly have to come for 2 weeks first to figure it out.  On holidays!

But if you are not spending too much and sraying in a 150 or 200 flat,  it will be enough.
As for me, I would not do it under 1.500 $

It's not quite good enough for $800 to live in PP. However, it depends on living is. If just yourself I think you should rent a house, 2nd floor house, or some type of house which only has 2 bedroom, kitchen, two bathrooms. All these will cost around $170 - $250 per month. If you want the really good one then will cost $300 - $500. The food it is not too expensive if you cook by yourself.

Then, if you spend wisely this will be live easy in PP.

The problem with the food is the following. .. if you can eat khmer food so count around 5 $ a day including a draft beer.
If you want yo eat western, it can easily be 15 or 20 $ s day. Think that 1L of milk cost more than 2 $ !

I only want a one bedroom appartment at a beach resort and not in PP , and i can live quite modestly ,I will have a cash lump as well as my pension but I just wanted to live on the 800 dollars per month if possible

So may be possible.
Phnom - Penh is much more expensive than Sihanoukville.