Non-immigrant Visa

Apologies if this has been covered elsewhere but a search didn’t turn up anything.

After having held a work permit while employed by a private company, I've since been asked to perform work for a Thai government agency. While no work permit for this job is in the offing just yet, the agency has in the meantime given me a letter to submit as part of a non-immigrant visa application. The letter states that my employment will be on a project-by-project basis, then goes on to mention that whether or not the work will lead to a full-time position can’t be said at the time of writing. It then asks for granting of the appropriate non-immigrant visa.

My first question: what is the appropriate visa? Would it simply be a non-immigrant B in this case? Or could it possibly be the non-immigrant EX visa for specialized work? (The work entails English-language writing and editing; not terribly specialized but they can’t do it themselves.) It doesn’t seem the non-immigrant F for official duties conducted with the government would apply here.

In line with the above, I’m wondering if the letter isn’t too vague and if a particular non-immigrant visa should be specifically requested, or if things should just be left for consular officials to determine. As you might have guessed, I have written the letter at the agency's request. They’ve just suggested the content and signed off on it.

Lastly, what’s the most that can be expected here, a typical 90-day non-immigrant visa with the possibility of extension? Or does the government-work angle change things?

Thanks very much.