We combined Preah Vihear, Koh Ker and Beng Mealea temples in a two day trip from Phonm Penh to Siem Reap. We rented a big car that fitted our big family of 6 for 110 usd per day (pay for own gas). It is so relaxing to visit temples where no other visitors travel. We slept close to Preah Vihear. Roads are fabulous everywhere now. No need for a guide in Koh Ker except maybe for Preah Vihear. You can climb Koh Ker pyramid temple via a newly built wooden staircase at the back. Fantastic experience and great views.