I'd expect the group to be something rather like the Fellowship of St. Giles. You don't declare your objective to be the extermination of a major power and then spend time in the sunshine. So, they'd be on the run a lot, worried about wizards blowing them up, or just hunters that wizards can buy with regular money. I'd assume that they'd get some under the table support from the vampires, just like the Fellowship gets support from the council. And I'd expect that if their secret got out, then the entire council would probably mobilize against them.
The powers could look something like this:
Defensive power, -1 refresh, (around 1.5 stunts worth of power)
Defend against magic at (conviction +1) (maybe should be conviction +2 instead?)
Hard to locate -1 refresh
Spend a FATE point to have a thaumaturgy ritual link against you automatically fail?
Or possibly spend a fate point for +4 defense against direct thaumatury?
Suppress magic, -2 refresh
You are able to start a (conviction+2) based 'grapple' against a wizard that works against their magical abilities, range 1 zone.
You still need an aspect advantage to start the 'grapple'.
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Remember, when designing defensive powers, that straight up immunity to magic costs 3 points (Physical immunity, -2 only against magic, -2 not-magic is easy to come by, -1 researchable)