Done Cambodia 2 years back; Am itching for Laos/Vietnam next, so if there's such trips planned, I'll be interested.
Organiser : please make sure your group are all experienced riders who have at least biked from Singapore to Thailand at least once before. NEVER allow anyone who's just gotten their licence just recently and worse still, just bought a new bike just prior to the trip to join.
Singapore to Thai Border is an easy but boring leg, so anyone with a decent bike should be able to make it. From Huahin to Bangkok, the traffic gets pretty wild especially with the heavy vehicles even way into the night, so do becareful. After Bangkok, the traffic should have thinned out and gets easier.
Entering Cambodia is where you have to be fresh & alert. Roads leading to Siem Reap just gets from bad to worse (unless they finally resurfaced it). Beware of broken bridges; you should be able to tell - local vehicles (if any is around) will just divert from the main road. Besides, you probably won't be speeding on these 'roads'. I only managed to go as fast as 3rd gear but only very briefly.
Once you reach Siem Reap, you're already an experienced rider, so I'll leave you to experience the other surprises on you own.
Things to note for your photographic equipment - Plenty of damping is needed; not a joke. You might want to have a cheap compact around to take photos while on the road. The dust takes way too long to settle and passing vehicles will just stir up a whole lot more dust.
One very important thing - Coffee drinkers. Pack enough coffee to last for the whole of Cambodia leg. You'll find it very difficult to find shops that sell coffee.