costs are on the website of the government - these are doing costs incl. gov fees - extension if you fulfil the requirements (4 employee under sso, capital etc) - 2k is monthly accounting ... not included yearly accounting and check balance
there are plenty like sunbelt, interactive who are pretty cheap - if u go with thai only lawyers is half price - but u might have understanding problems - there are plenty
of law firms out there - mdlegal as well for example.
what kind of paperwork ? for setup the company or later ? lawyers have an english copy of the company setup papers - or do that for you. Later you can do english contracts for example — only thing in Thai will be from governments - get it translated or have an accountant who speak english as well - it might make sense to have a thai manager as well.