An organised tour will make it easier as they will have it sussed and people on hand to help and point you in the right direction, re cash, most places will take a card. But you can take cash out of ATMs even in Cambodia
I presume you will be heading to the standard tourist places? Siem Reap, Ha Long bay, Hoi An, Hanoi, Saigon, Bangkok, if so atm and banks are easily available. It’s pretty cheap compared to Europe so you don’t need much.
All hotels will do washing but there is often a place just outside the hotels / resorts that will do your laundry cheaper than the hotels (usually the grand mother of the cafe / bar owner will do it for you for a bit of extra cash.)
Re mobile sims, I tend to use my UK phone when abroad, most places have Wi-Fi so if I do need to make a call wait until there is Wi-Fi available
I loved Cambodia although Siem Reap is a bit seedy but fun. Hanoi is probably my favourite city on earth, Hoan Kiem Lake at the weekend is great. I am sure you will have a great time.