If you're returning to your home country each year, probably best to get a Multi-Entry Tourist Visa. This can give you a potential nine months in Thailand with two strategic border bounces.
If you're going through Sydney consulate apply six weeks before your planned departure date. You will be required to give your flight itinerary, so yes you can get your ticket anytime. The sooner you get it the cheaper it will be
Assuming he's UK passport holder, doesn't need a visa. Can enter visa free for 30 days. With at least 15 days still outstanding on the entry stamp he can apply for a Non-O (over 50) visa. Conditions for this vary from office to office. In Bangkok they want to see the 800,000 baht in a Thai bank account and a 12-month lease. Once he's got the 90-dayer, he can apply close to the expiry for a 12-month visa extension with similar criteria as the 90-dayer. There are no Multi-Entry visas issued in Thailand. He simply gets a Multi-Entry Re-entry Permit
The OA requires ongoing insurance, but from the rumours emanating from different areas it seems all retirement visas will require insurance in the future under the reforms