Get the train from Bangkok to Nong Khai. You can book a rent a car to pick up in Nong Khai, drive along the Mekong to Chiang Mai and then wherever. You can arrange to leave the car virtually anywhere you want. I have done this several times, stay wherever you like for as long as you like. Or you can get the train to Nong Khai then just jump on buses to tour around. You won't have any problem finding public transport in the north and you will have a great time.
If you don't need the beach have a look at Nong Khai. Nice little town on the Mekong/ Laos border. Quiet , friendly and good shopping. Railway to Bangkok when you need to go. Lots of nice villages along the Mekong a short drive away. If you want beach then look at areas between Rayong and Chantaburi. Many beautiful spots, fresh fruit and seafood and a lot cheaper than the more populated areas. But seriously, for someone to answer your question you will need to give a lot more info, ie, do you want western food, a high level of healthcare, public transport, live in a expat community or a thai village? The possibilities are enormous.
many people, like myself are claustrophobic, hate being in crowded rooms or standing in queue's. I really don't like sitting in taxi's for hours either. I would much rather pay an agent to do all this for me.
It is far cheaper to rent than it is to buy in SEA. Negotiate to pay a year in advance for 10% less than they are asking with no deposit. Landlords can never resist a wad of cash in the hand and you won't see them for a year.