Have plonked money down for air tix to Cambodia this December, so will now have a host of questions that need answers

hehe.
Q1. If we're thinking of taking a bus from Siem Reap to Phnom Penh, how easy is it to get tickets, given that it's peak season in December? Can we just fly in to Siem Reap, go look for a bus company and get seats for three days later? If not, can I book bus tickets online?
Q2. How good are the buses? The deluxe bus services run by the govt bus company in Thailand are superb. Are there services of similar standard plying the Siem Reap-Phnom Penh route? If not, what can I expect?
Any help will be much appreciated. Thank you!