Acceptable purpose of visit for non O based on pension?

I am travelling from UK to Thailand soon with my elderly father and Thai life companion for a ninety day stay. The main purpose of this trip is to help my father to decide whether to move to Thailand full time next year with us. I could apply for a ninety day non Obased on my marriage, and my father could apply for a ninety day non O visa based on pension. Butmy fatheris not entering with intention of retiring there on this trip, and I am concerned that he cannot validly state ‘retirement’ on the application formas the purpose of his visit, and am not sure whathe should say if we are asked about purpose at the immigration desk on entry?
Should we apply for 60 day Tourist Visas for this trip, with a view to extending them for 30 days, or am I being overly cautious?