In the opinion the 5th edition core book is a masterpiece, well above anything else published for Ars Magica, including the 5th edition line. You can play a whole saga using that book alone. The only thing where it is lacking is a description of the hermetic society, really, with a tribunal map and a few paragraphs that describe what a tribunal looks like and how the diverse tribunals diverge gfrom the norm. And the bestiary, that is somewhat short.
For other stuff, I would also go for a tribunal book. As suggested, it removes a lot of work from the alpha storyguide and helps to settle the atmosphere of the surrounding region. Since you can basically ignore the numbers of any of them, you could get hold of any of the tribunal books in existence and it will do fine. Heirs to Merlin (England) or Guardians of the Forests (Rhine; germany, poland + holland) would be my preferences since they are quite the standard settings. Heirs to Merlin is 100% compatible with any edition, since it contains no number crunch at all (it is pure background info) and quite good at it. I have a weak spot for Lion of the North (Loch Leglean tribunal) but it is old (3rd edition) and I hesitant to suggest it.
You do not really need mystery cults to play a MC magus. The rules in the core book are enough to play around their concepts and virtues and flaws.
Download the book of mundane beasts and the free material from the atlas website.
Search for Hermes Portal. That is the 4th edition ezine of Ars Magica. There are hundreds of things you can take advantage from it. The whole collection is online for free. Its follo up ezine (subrosa) is a payment one, but you could cinsider getting a subscription given the quality of the product.
If you need adventures, the Cliffheart saga has quite a few of them:
arsmagica.it/Cliffheart/risorse-en.htm
For sagas, try Project Redcap (www.redcap.org)
I am specially fond of these 3: got lots of ideas myself from them:
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TRIAMORE SAGA
myweb.tiscali.co.uk/thelemur/ars/ars_magica.html
You would need the 4th edition book "Triamore" to fully understand this one, but it is a very detailed covenant and saga.
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SAGA PALATINI
arsmagica.andrewgronosky.us/wiki ... f_Palatini
And the rest of Andrew Gronosky's material in that wiki. Great stuff.
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SEVERN TEMPLE COVENANT
severntemplecovenant.co.uk/
OLd saga, amazing reading. Weeks worth of material
For starting a covenant, I would say "don't". Been there, done that, several times. Failed generally.
New playersto Ars do not control the setting, the capabilities of their magi, the nuances of the Code of Hermes and generally are an elephant in a glass store when it comes to itneracting with the setting. Given that, making them junior magi in an older, stablished covenant can be a good idea. The older magi can mop up total disasters and act as (unreliable; never trust them) cavalry to come to the rescue from time to time. If you want to give them plenty of freedom, starting in a WINTER covenant can be a good idea. THere are some AWESOMELY powerful magi around, but they take care of their own stuff. They might even kill you if you annoy them too frequently. There is one of the remaining magi that is less mad than the rest, and is the one that recruited the magi. When he is not in twilight he can give some advice and order them around (read: act as a counselor and adventure giver). Ungulus, in Stonehenge, can be such a covenant. PLenty of resources, plenty of disappeared resources and a really weird place to be at, even if the power gained is great. Stick and carrot
Cheers,
Xavi