Thailand offers a very generous 30 day visa on arrival for most countries. Other countries can apply online for an evisa or get the visa upon landing.
Thailand is considered very safe and very tourist friendly. There can be some parts in the south near the border or also on the border with Myanmar that can be dangerous but there is no reason to visit these places as a tourist.
Really depends on where you are but it is cheaper compared to western places. End of the day, it’s also going to be all about how you travel. You can do it very expensive and choose fancy restaurants and hotels or you can choose street food and budget guesthouses.
Credit card acceptance is very high in the major cities and on islands you may have to sometimes pay a surcharge of 3-5%. ATM’s are readily available everywhere.