I travelled through Cambodia and Vietnam as a young single female traveller and had no problems so just keep your wits about you and you will be fine. Make sure you have travel insurance if you;re worried about hospital fees. Trust me I just had to use it to the maximum in Thailand, (bad timing and a freak bystander injury later, the $200 AUD insurance cost more than covered itself).
The roads in Cambodia are bumpy and dusty and not well kept in lots of places but thats just the nature of the country. Make sure you go on an actual bus, not one of those over full station wagons and you'll be fine Plus it's a great way to see how people live there as you will pass lots of rice fields and nice scenery.
I've been to Thailand 3 times (Phuket once, expensive and dirty there) and have had the bad luck of being robbed but just make sure
a) TRAVEL INSURANCE
b) Always lock your belongings up in the safe before you leave your room (Thats how it happened to me, first timer traveller stupidity!)
c) When out, keep your bags on you at all times
d)Back up your photos regularly incase of lost/stolen camera!
e)If alone, keep the drinking to the minimal and do not piss off the locals.
Even though I've had bad experiences in Thailand, I still love the place and the people