Have you ever heard of an avid bb7? Cable disc, can easily compete with hopes on the power front.
And what rear brake is Ali C (and many other top riders) using? Yep, Vee's.
A good rear brake is a must, but all you'd really need is an avid sd7 (cheap) with a slick cable and decent pads, or a second hand magura with decent pads. Both of those are very, very good brakes and very affordable
As for the bike, it would work for learning the basics on, but you'll quickly become very limited by it, and at that point, if you still like the idea of trials, then you could upgrade to a 'proper' trials frame, swaop over what parts you bought for the previous bike, and continue to improve
Matt