http://www.tartybikes.co.uk/product.php?product_id=10276&category_id=47
I would personally recommend something like that, going on your MTB background.
It's also worth looking out for other stock bikes on Ebay.
The first thing you ought to get the hang of is trackstanding.
Then getting comfortable with hopping on one spot and rocking on the bike to keep your balance, all essential techniques!
Then you can work on backhops, then pedal-kicks, and more.
http://www.trashzen.com/
Trashzen is filled with how-tos for techniques on trials bikes, but you'll also find some in the trials-forum WIKI, found here. The TF wiki also has mechanical tips, should you need them.