A tourist visa from the UK costs £30. However as of yesterday they changed the rules and now UK passport holders get a free 60 day entry, which is extendable once for 30 days at a local immigration office, so there seems little point in buying a visa.
for Russian passports the 60 day stamp wasn’t extendable nor transferable to a different visa type. We’ve yet to understand what the new entry stamp of 60 days will work.
Usually easier to apply before in terms of being able to set up a bank account and stuff, but both options will work fine as long as you can get the bank and financials in place.
For a consulate or embassy outside of Thailand then you should be fine with a temporary address. Depending on the office then you probably will be ok for applying inside of the country for the initial Non O, but to get the year extension off of either option you’ll probably find your office will require a long term 6 month or 12 month contract.