Start with Travelfish. That may be all you need. I just came back from my upteenth trip, this one focusing on northern and western border temples. Be careful where you stay in Siem Reap and avoid the so called Pub Street area. Wat Bo area remains ok.

I'd spend several days in Phnom Penh, which is undervalued. I'd avoid Kompong Som/Sihanoukville, which used to be good but has been overwhelmed with gross Chinese development.
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Rich