You mentioned "sear". Think I'm going to be a fly in your soup. Sorry!
Almost certain that you can't convert a sear gun to select fire. It would require a navy lower, that contains the select fire mechanism and the sear built in. That can only be installed on a registered receiver gun. Kind of answers part of your second question. Marks are on that part.
Again, I'm not even close to being an expert and can possible be completely wrong. Simply deal in the stuff on occasion and shoot whenever possible. Would strongly recommend that you post your question on subguns.com and get a more precise answer or talk to someone who has actually built one of these things, legally. Been around these things long enough and have never seen a successful select fire sear gun.