Know your capabilities. Know your limits. Know the market.
What do you want it for?
If it's around town - swallow your pride and get a small scooter. If it's off roading in the provinces - it's a dirt bike.
If you are going for a popular item like an AX-1 or a small scooter- expect that it will be stolen. Reason is these are closer to the ground than the slightly larger other off road motorbikes.
Remember that if you are taking it off road - it will get scratched and have bits knocked off it so a new bike will quickly deteriorate. Equally when you are buying a second hand bike remember what it might have been through: crashed and hammered straight, dropped in a river, left out in the rain etc etc Check how many layers of paints it has had. Check under the seat/tank for patches. It will have second hand parts, it will probably not have a service history. It may have been stolen. It may be two different bikes. It will have had a steady diet of bad fuel which anywhere but major cities is hard to avoid. In the provinces - fuel is kept in plastic cola bottles and the person filling your tank is usually smoking... It will not have had the fuel or air filter changed in a very long time. Caveat Emptor!
If you drop it when you are off road - you will have to pick it up yourself - the larger the bike the heavier it is. A 250cc model is quite sufficient for 95% of those riding motorbikes here.
Can you maintain it?
Getting original spare parts is a nightmare and often they have to come from Singapore. Getting parts for the popular items - Like XL250's, Honda Degree, Honda Baja's and Suzuki Djebels is not too bad as there's quite a few around. A new race bike or a BMW may have problems. This is due to the tax laws on importing spares for bikes over 100cc. I met one man who had a Suzuki GSX 600 - a racing fared bike - he sold it after a month - the Police stopped him on average twice a week. The more pricey the bike looks - the more attention you get. The are very few sports motorbikes in Cambodia. Remember they are close to the ground so a good target for thieves.
Maintenance
A Khmer run bike shop will probably NOT carry original spares (but they may tell you they are) and their standard of work might not be what you expect. I've heard reliable stories of bikes being put in for maintenance and coming back with some original parts removed and dodgy second hand parts substituted. Currently there are two or three trusted bike shops in Phnom Penh - the one which is regularly recommended is "The Bike Shop" which fixes bikes to a reported high standard. Remember local mechanics learn from their parents/elders so the knowledge they have may only be suitable for one make of bike and may be out of date - but that's not to say that there are no good Khmer mechanics out there. Start by asking if they have the manuals.
The more complex a bike and the more electronic parts it has the more can go wrong. But electric starts in this climate are an god send.
Buying it: Trust no one -- question everything
Only trust a true professional and always get a second opinion. Best things is to see it running or buy from someone you know. Always find out if the tax has been paid - if not it may have been stolen. It should have a number plate and you should get this checked with the ministry of transport. And it's your only way out when the police stop you. Carry the ownership card because in any accidents if you do not have this then you loose the right to claim compensation against the other driver - if it was his fault. For about $50.00 a year you can get third party insurance. This is a good idea given what accidents usually cost and if it's a serious at least you have some recourse.
Riding it: be paranoid
Assume everyone is out to kill you in Phnom Penh - motos, kids, cows,dogs, land cruisers, other drunk riding westerns, police who hit you in the chest with a baton when they want you to stop. Don't expect lights/horns/mirrors to be used. Assume drunken ignorance from all. Out in the provinces expect to find dogs lying on the road, cows, goats, water buffalo, missing bridges with no warning, people standing in the road, truck ruts three feet deep, landmine fields. Roads blocked completely for weddings right after a sharp bend. If you see a branch in the middle of the road - it might be a sign there is a hole under it. Puddles can mask a chasm. Bridges that are two pieces of stick. Ferries that are two canoes with a pallet over the top. (Go slow and ride in groups - go with experienced people) Don't ride in the provinces in rainy season. Take a GPS and know your road. Most provincial Khmers do not travel outside their villages and signs in English are almost non existent - few can actually tell you which way to go if you are lost - that's if you can speak Khmer. Take a survival kit in case you get last - water, spare inner tube, levers and a basic tool kit, mossie nets - all a must!
Get a decent helmet - the ones available here are pathetic. Riding Without one is suicidal. Riding in shorts and t-shirt is stupid. Skin Grafts hurt and they don't do them here. Emergency helicopter medical evacuation is at least $20,000.00 and then the medical bills start.
Get boots and a spine protector if you are going off road. Again there's almost none available here. But Paddocks in Bangkok carries a good range including light weight armour jackets that are easily wearable in hot countries. Wolf racing do a good line in protective boots. There are some excellent armoured Japanese jackets which even go up to a 48" chest.
Get a good lock. If you are really stuck - a padlock through the holes in your front disc lock might do it but locally bought ones seize up regularly.
Riding drunk/under the influence of drugs is just as suicidal as riding without a helmet or in shorts and t-shirt.
Whilst many do not obey traffic signs - it's a good idea to do so.
Crashing it - don't be alone and have insurance
Just about everyone does. Have health insurance and get treated in Singapore or Bangkok - there is a company called Fig Leaf that runs an expat health insurance policy - not sure of the exclusions.
Enjoying it: Simple
Having said that riding through villages that rarely see westerners and camping out in the middle of nowhere is excellent. Spending your dollars in tiny poor villages makes a difference and the entire village will turn out to see you - especially if you make a prat of your self and over balance in the middle of the village. Many hands will help you pick your bike up while your friends take numerous photos to hand round at the bar when you get home. Riding past mountain ranges that take your breath away, along deserted country side, trying local rice wine, sauntering past paddy fields and finding those hidden places you can tell your mates about who stayed behind. Spending your weekends soaking your mates through running through dirty puddles - good days - good pictures - good memories. Preparing for the next weekend away or the next Sunday run - getting out maps in the bar, seeing other people's scars/photos and amateur videos of their last runs. It's all worth it. Just stay alive.