One last suggestion . . .
If taking the bus from BKK-REP does not appeal to you, you could also consider the following - fly to PNH using Air Asia (~$100 OW) and then take a boat to REP. There are both high speed and slow boats up the Tonle Sap River to REP through the Cambodia countryside. It's a pleasant way to travel although it would add about 1 day to your travel itinerary.