Sihanoukville

There will be a group of four of us in January and we have enough money to spend on comfort:) thank you

There is an airport in Sihanoukville, but they fly to Siam Reap. The quickest and probably easiest Ho Chi Minh City would be to go by bus or by car (taxi) in Phnom Penh and fly from there. Vietnam Airlines offers daily flights to Ho Chi Minh City to Phnom Penh: http://www.cambodia-airports.com/phnompe ...
I think it would be a little more comfortable than the water buffalo. You can also take a bus to Ho Chi Minh City, but this is not always the safest. Have fun.

take a bus every day. more info on www.saigontourism.com.vn

Good luck!

Water Buffalo