I've just begun my year of postulancy with the SSPX Third Order :) It's explained fully here: SSPX Third Order

I honestly don't know about attending other mass centres, Belloc, sorry! I know that, as an SSPX TO member, one should not attend the new mass. The Rule states that members should go to Mass every week (just as for any Catholic!) but more than anyone the SSPX also knows how hard it is for some people to get to the TLM every week (myself being one of them) and so this is not an obstacle to membership.

If you can't get to the TLM as often as you would like, I would say that the SSPX Third Order would be an excellent choice because it will help you with those extra graces if you don't have regular access to the Sacraments. It was set up to "secure for those souls living in the world a school of sanctity" and the Rule is pretty easy to follow- nothing extraordinary, just the basics really, as far as obligations goes.

There's more info on the link I posted, and you're more than welcome to ask me any questions you like.