Generally many insurance companies don't cover pre existing conditions or if you declare these conditions there is an additional premium and limited coverage..
Depends entirely on your individual circumstances, age, financial situation, income from external sources outside Thailand (if any) so quite a range of things to consider.
You can get this on arrival but it will slow down the entry processing because you have get the visa from a separate immigration counter first before you proceed to the actual immigration officers for border entry processing.
Sometimes visa on arrival can get busy so you may have to line up with many other people to get your visa.
It's easier to apply online for a eVisa before you travel and then just proceed straight to immigration processing on arrival..
The eVisa is for a single entry up to 30 days entry into Cambodia .
And it's valid for 3 months so you can use it to enter anytime during a 3 month period....
Depends on your passport country but most require a visa but these can be obtained on arrival or before you travel via Email through the Cambodia government visa website..