Have you checked into the cold feed TIG machines out there yet? They are a wire fed TIG. Might not be bad for an automated process. I looked into them for a project where I'm the tech - CWI, but it wouldn't work for what I was doing. The FCAW stainless is a good choice also. I guess it all boils down to what your facility is used to using. How long will these things be when you are done? I assume your 3G will be in the upward progression. Oh, make sure your vertical seams are staggered.
As for a supplier, Try big blue, (MILLER) They can surely help in that department. AWS D1.6 does have some prequalified joint details, see if any apply to what you are doing. Check section 3, part G, prequalified tubular joint details. 30 Deg. should be fine for the Mat. thickness you discribed (1/4"). If I helped you, OORAH. If I only suceeded in confusing you, sorry. I'm sure you will get responses from people out there that might be better informed then I.