In the example given for advancing Darius, he has 20 XP left at one point, and uses it to buy a version of a spell. My question is, how? I didn't see that as an option and assumed that it had to be done with lab activities.
It is completely legit: see the box on the top of page 29 in ArM5, under bullet point 8 (the first one 
)
...only during character creation, I might add.
If your a skilled player this is a great way to start a focused character or indulge in Ar's nigh-perfect magic system. For newbies, I'd stick to the spells detailed in the core book.
Aha! OK, good to know, that can speed things up considerably as I'm designing some old magi.