All this talk of longevity specialists... the resident longevity specialist in our saga developed longevity potions with lab totals in the early 100s (mine is 117, his 124 IIRC) in just his third decade post gauntlet. (Admittedly, we are in very good conditions - a generous saga of lab work.) As Agnar said, cooperation can increase these totals further. And then there are deep inner mysteries for the really old magi...
The requirements and demand are just not so high IMO for extreme specialists to be so valuable a commodity. By the cannon, few magi live past age 200. To age, you will need to roll 10 or more on dice+200/10-Bonus. Let's say you want to insure yourself against aging on a roll of 12 by age 200: the Bonus needs to be 22.
A rich magus should have -2 in Living Conditions, so we need +20 from the longevity ritual. (I'll ignore the possibility of arranging positive Health modifiers using rules from Covenants.) At +1 per magnitude, that's a lab total of 100.
A lab total of 100 isn't that hard to reach, especially for two old magi working in concert. Take any mediocre expert in the field with a suitable focus, he will likely have something like both Arts at 20 and Magic Theory 11 (that's the hardest to get, I think). Add to that your own stats and those of both or one of your familiars, and you're basically set:
20 Creo
20 Corpus
20 Focus
11 Magic Theory
5 Int (through spells; he's a Creo specialist!)
5 Art
3 Aura
7 your Magic Theory
5 your Int (why buy only longevity potions?)
5 familiar's Int
7 familiar's Magic Theory
108
Room to spare.
That's just one constellation, there are plenty of others. All you really need is a Creo or Corpus or anything-else-appropriate specialist (for the Focus) to spend the time getting his Magic Theory high enough, and you've got the bulk of the needed sum. The reminder isn't that hard to arrange for. And if you're short a few, up your standard of living or your laboratory's Health, or invest some more pawns of vis.
All that assuming longevity is still relevant, which it isn't if you've followed a Criamon path to its repose, or a Merinitia path to faerie, or a nature mystery to become a natural guardian, or (when TMRE comes out) the alchemical path to immortality or the theurgic path to divinity, or the Forest Path to immortality, or...
Sorry, IMO longevity specialists are neat and all, but aren't THAT rare and do not call for SO specialized training and learning.
Yair