There are several documents on this page detailing what you need. According to section 1.27 of the Infrastructure criteria, all First Division and A Championship teams need a 500 individual covered seats (or an approved plan to get this in place during a certain time frame).
Also, on a sidenote, do Mervue plan to stay in the First Division next season? I know during the off-season there were rumours that they might resign from the First Division at the end of this season. That might open up a spot for the highest placed team in Group 2, even if they do miss out on 3rd.