When I did mine I went down to the local court house and they printed out a statement to show I had no active cases. It wasn't a document a long criminal history or even extensive, it was only valid for the local municipality. The Thai Government will accept almost anything.
My Dentist charges 39,000, it takes a few trips and about 6 months. 3 months for the mouth to heal after the extraction and another 3 months for the mouth to heal from the implant of the post.
I used me driver's license. It's been a requirement for a few different visas, for awhile now. It's also the reason I feel comfortable applying for visas while in Thailand.
bahtsold.com is a pretty good site for finding places. Also, I would recommend walking around neighborhoods you may want to live in. Several/all condos have a juristic office (management office), which will likely have current available rentals. Last resort, craigslist
In many cases a visa gets you a 30 day stamp that can be extended +30 days, but that depends on your nationality, so you will need to check the Thai Embassy [in your country].
I haven't ever gotten a retirement visa, so I only know what I have been told. I have a friend that gets retirement visas (normally you just renew but he let his visa lapse). He told me that the insurance requirements were different so it was his preference to apply for the retirement visa in country.
I am not sure of other differences.
Differences between METV and retirement? The list is pretty long but basically METV gets you up to 9 months if you work the system, retirement visa is 1 year, no working the system.
"Working the the system" - stamp-in 60 days, extend 30 days, leave/return stamp in 60 days, etc. You only need to leave for 5 minutes (I go to Cambodia and return same day). Effectively you can get up to 9 months. Usually people will choose this option if no other visa option is available.