Are you'll want to pay the penalties for that so soon into the character's life? It becomes so very expensive to progress after Becoming (or any other form of immortality, actually). In that particular case, the restrictions on spontaneous magic seem especially painful to me. Also, when speaking of vis, remember you'll need to power the three rituals with something. They won't necessarily be Creo, but you'll need to come up with something and, if that's your goal, it might be best to start planning for the TeFo combo you'll be using now and start securing the resources.
FWIW and pending SG approval, I was planning to start with a Personal Vis Source (Creo). I hadn't really given a thought towards a Longevity ritual when I added it; it actually just made the most sense with the source I had in mind, but it should definitely cover my needs in that area.
I'd like to see a reasonably diverse collection of sources for our covenant. As has been pointed out, with Merceres in the group, we should be able to trade at reasonable rates, but it is still nice to have a nice range available natively, as it were.
It might be a conflict but the concept of need before greed would state that those who were still looking at aging issues have a greater need than those who are looking for Cr vis to, say, whip up some better lab equipment. If an immortal magus had a need just as pressing, I'm sure it would be considered with equal weight, but I think he'd otherwise be waiting for his turn, just like those who had already completed their conventional Longevity Rituals.
In the same vein, I'd expect my magus, with his own source of Creo, to be bypassed when it comes to handing out the Creo vis from covenant sources unless he was doing something for the entire community that was pressing enough to warrant the allocation.
Al that might imply that I lean towards a distribution-by-Council-approval method. I'm not really sure that I do, although I don't think I'd argue against it either. I think we'll have to plan out other details of the covenant first.
A vis salary and/or stipend is a nice, orderly, and conflict-free solution that can save problems later. The difficulty is establishing what those rates are in the first place, and it relies on there being enough to share that everybody's equal portion is really something meaningful, in terms of both quantity and type. 2 pawns of Perdo vis a year isn't going to do my magus (with his Pe of 0) much good at all.
If we don't end up with enough vis of sufficient types (and given that there are 7 magi, I don't think we can assume we will until we get further along into covenant design), we may have to go with a council-driven, project-based allotment scheme just to make sure everybody with an important project has enough to work with.