Wanting to do some of the early Telvanni quests to get some cheap spells (namely an easy way to get Mark and Recall) but the character's better-designed for being a member of House Redoran (heavy-armoured spear-wielder, what magical skills he has are just for mobility or the like). Will I be prevented from joining House Redoran as soon as I sign on with House Telvanni or is the cut-off based on a particular rank?

As soon as you join you are blocked from joining any other House. The Houses are constantly fighting; if you become a member of the family, you are family, and no other House will accept you.

If you're in Balmora, in the basemant of the shrine there are two npcs; one sells mark, and the other recall, iirc. I don't think they are that expensive, although it has been a while since I've played.