I only play with some pieces of the power 9 (no 10 unfortunately) because I picked them up in the Collector's Edition set - which might be a good idea for you if are strictly playing casual. They run about 20-30% of the full value of the regulars.
And if you playing multi-player the speed demons lose a lot of power.
I've played with the whole 9, and in multi-player I've found that Black Lotus and Time Twister aren't even close to 1v1 level unless you are playing some extreme combo deck. Time Twister definitely can't go into just any blue deck, and as Hetemti points out about Black Lotus, after turn 5 you'd wish it was something else.
The Moxen are nice, but far from necessary.
Ancestral Recall is the best card ever, whatever the format.
Time Walk is always a good card. At worst it's a cheap cantrip/untap all lands. At best its potential is unlimited. I actually enjoy playing Time Walk far more than I do the Recall.
Library of Alexandria I'd love to own. This card has drawn more cards for my buddy than I'd like to admit, and if you don't get a turn 1-3 Strip Mine/Wasteland in response to it, it will own you. Sometimes late game you can't draw with it, but it doesn't really matter b/c its still providing mana. Do not underestimate the power of this card, it is beyond efficient.
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All this said, I could definitely get by in my group without the power 9, and in a lot of my decks I do not play any of them.
I will say, before you shell out the cash for the P10 I'd take a look at the Type 1 restricted list and pick up your single copies of necessary cards on the cheap.
Ones you must own
Sol Ring
Strip Mine
Regrowth
Demonic Tutor