It doesn't hurt to have spare documents handy - even a map to your house ... I have been ask that every time on a retirement extension ... but it is quirks of each office and each officer... Spare docs in a separate folder can save a go back trip.
If the Bangkok Bank ATM / Debit card is labeled Visa it will work at ATMs in neighboring countries. If it is labeled Union Pay it will only work in Union Pay ATMs - and there are some Union Pay ATMs in neighbor countries
Elani -- just for little clarification. The B Visa is for Business (not titled 'Work'), however it is intertwined with a Work Permit. In most cases the Non Immigrant B Visa is issued by a Thailand Embassy/Consulate. The Thai Labor Ministry issues the Work Permit. In the B Visa application process a person must have at least proof of a successful Work Permit Application - usually a type of receipt called a WP.1 or WP.3 ... This WP.1 or .3 Application / Receipt will generate a Labor Ministry Official Letter. These Labor Ministry documents must be attached to your B Visa Application at the Thai Embassy / Consulate outside of Thailand where you apply. There are other documents that must be provided too but mostly are related to your employer's company. You will supply copies of all pages of your passport in the B Visa Application with is for a Non Immigrant B Visa 90 days and proof of your University Education. After award of the B Visa you can later get an Extension of Stay based on the B Visa or you can get a Multi-Entry B Visa for one year which requires a Border Bounce (Border Run for an out and in stamp every 90 days. These last two options vary depending on how your employer handles the situation.
There are some variations on this entire B Visa / Work Permit process - but the process I just described is a very common method and sequence used.
Also - if others have corrections to my comments - then I will be pleased to stand corrected.
A single entry tourist visa SETV is a 60 day visa - not 90 day. You can add a 30 day extension at a Thai Immigration office for 1900 baht. With meeting requirements you may be able to get a single entry Non Immigrant O Visa 90 days.
Your original question about having no established residence... It is easier than you may think. Live where you want to ... but in the same area you can do something like striking up a relationship with a Guesthouse owner/manager - book a room whether you stay or not... In this process get a receipt for your stay. You may try to see if the Manager will write a receipt for a longer term - for a modest inducement. Nothing states that you have to live in a Condo, leased Bungalow or Apartment. A Guesthouse or modest hotel will do nicely.
Going to Vientiane, Laos to apply for a Non Immigrant O Visa 90 days at the Thai Embassy there would be a good first step. Then upon returning to Thailand you apply for the Extension of Stay based on being over age 50 / Retirement at the local Immigration office. You will need proof of meeting financial requirements for both the initial O visa and the Extension but they are both the same. For a lot of people the simplest method in proving income from outside Thailand of the Equivalent of 65,000 Baht a month. You can get an official form letter - Income Letter / Affidavit at your own Embassy in Bkk. Proof of income to the Embassy depends on the policy of your Embassy. It all sounds a bit foreboding but taken step by step it is not all that difficult