I have noticed that it is possible to find old Ars Magica supplements like Mythic Places or More Mythic Places on various websites, at low prices (less than 10$/£/€).
As these seem to contain interesting story ideas I considered buying them, some of my players having asked for more adventures, and more loot. However I would like to know if all the stats (for PNJs, monsters, rewards...) are compatible with those of the 5th edition?
Search for "Ars MAgica Adventures" on the net. There are quite a few. The ones by Kevin Hassall tend to be really good. Durenmar has quite a few of those, and the saga of CLiffheart (reachable through the eire links of Project: redcap) has an extensive collection of adventures.
The 4th edition adventures are pretty much compatible as well. Festival of the damned, ghoul of saint lazare, faerie stories and a few others.