I would suggest that you definitely book your bus transportation ahead of time. Its easy to organize your trip and book / buy your tickets online on Viazul website. I saw many disappointed travelers in Santiago de Cuba as they were not able to hop on the bus 2 years ago. They did not have tickets. We got ours on Viazul website ahead of time. If you do not speak Spanish just select English on top right corner on Viazul website.
Casas can be nice and not so nice regarding how it looks etc I would arrange as well casas ahead of time. If you do not you will be able once you arrive with a bus to particular city / town in Cuba to talk with people who have casas available for rent at the moment. There is usually plenty amount of people who sell casa accommodations. Only problem is you might go with them but you do not like that casa and than you need to look for another one. Carrying your luggage bags around town on hot day is not fun.
ATMs depending on the town and city might not accept your Visa card. Debit cards are not accepted in Cuba! The best place to go and pull money from your Visa card is CADECA (Casas de Cambio) where you can exchange money or take money from your Visa card. You will need passport as well your Visa Card to do this. Every bigger town have CADECA. Sometimes you do need to wait in line to get served. I personaly never have too much cash with me .. just enough for a few days and once I need more I pull from my Visa card at CADECA.
If you have more general questions about Cuba you can post it on our Cuba Forum so everyone can see and help out