The current Bundle of Holding is for ArM4 books. I'm afraid that you have to opt to get Heirs to Merlin, but if you beat the threshold you can get a lot of good books as well.
The offer is open for another eight days, and I'll try to remember to unsticky this when it ends.
I've just bought a physical copy of the 5th edition core book. I'm wondering how useful the 'beat the threshold books' are for running a 5th edition game? Also is there a problem with Heirs to Merlin? 
If you are starting a saga, most of the ArM4 stuff offered is great, regardless of the edition you are playing in. Triamore is a great covenant, bit as a PC covenant and as a NPC one (ally or enemy) and can easily be mved wherever you have decided tp play. Same with faerie stories and festival of the damned. We have ran both story arcs with good results ourselves. A medieval tapestry is a great source of characters and so is the bestiary for, well, animals both supernatural and mundane with adventure seeds.
Nice option to get older material if you do not have it.
The Wizard's Grimoire Revised Edition won't be much use, although some bits are still good for inspiration, but as Xavi says the other material is eminently usable. The game statistics won't quite match up with the rules you have, but the necessary revisions should be obvious.
It's a work of deathless genius. 
Thanks for your replies, I'll pick up all of them
The bundle will be closing in a few hours. Last chance to pick up a bunch of ArM4 PDFs at a really good price!