I'm not sure but you'd be best booking something for a night or two and looking around when you get there. There are some lovely parts of Siem Reap and some pretty crap ones, like every town. I'd imagine most places will do you a long-stay deal if you ask.
Are you sure you want to stay in Siem Reap for a whole month? Apart from Angkor and surrounding temples, there is not a huge amount there. You can take the train/boat/bus to other towns to visit too?
I was there for a few days and liked it. Check out the Butterfly Cafe, lovely.....
