Hello
According to the MacGregor site, they do not claim Sangsters as a sept, as you can see here.
Welcome to Clan Gregor
If you live outwith Scotland, wear any tartan you choose - there are no tartan police! Personally, I'd avoid the Buchanan though.... far too bright for most peoples' taste
You should remember that not all Scottish names were a clan or even sept to a clan - especially if they were based in the lowlands.