Moving, Buying Guesthouse, Need Work Permit Questi

I have never done this before so please bear with me as I struggle along.

I am an American with an unmarried partner who wishes to move to Thailand and retire there. We have money to buy a building with some property attached. We have heard a little bit about an option # 3. A 30 year leasehold on land with or without a building on it, with an option of further periods of 30 years lease. So far, I have not been able to find an exact explanation of how this works or what type of Visa or immigration proceedure we would need to do this.

We had hoped to buy something that would permit us to rent 2-3 apartment units to visitors while living in it ourselves, to support ourselves in retirement. Is this possible? If so, what kind of legal status would we need to apply for and would we still need to leave country periodically to renew something?