Dave,
Up until a few months ago I hadn't owned a mountain bike since my early teens - all my cycling had been on the road. I was looking to mix things up a bit and was inspired to try going off-road by a few blogs:
Dave Barter's:
http://phased.co.uk/
Ian's:
http://ianbarrington.com/
Aidan Harding's:
http://www.aidanharding.com/
There's a wealth of info and some really useful insights on each of them - I recommend reading through the old posts. Aidan's kit lists for various events are comprehensive and certainly worth checking out.
That said, here are a few of my observations having now put in some off-road riding.
1. I went with a rigid steel 29er - and when I first built it up and rode it, it felt like a monster truck. It takes some getting used to.
2. Even 580mm flat bars feel very very wide when you're used to riding 42 - 44cm drops. It may take some time to find bars that suit you.
3. If you are riding a rigid fork on rocky terrain, a high volume tyre at low pressure provides passive suspension. That doesn't mean that it becomes a quilted carpet ride, but it definitely helps a lot.
4. Tubeless with sealant is simply fantastic.
5. Tyres that suit some riders may not suit you. I went with Ardent TR EXO because it's well liked by many and seemed to cover a lot of different riding conditions. However, it's not working for me - a lot of my off-road riding has been very muddy and the heart-stopping moment count per ride is a little too high! I am sure some will say that my handling skills are not good enough - and they would almost certainly be right. But, nonetheless, I'll be switching to something better suited to muddy conditions. So, be prepared to experiment a bit with tyres.
6. Cable disc brakes take some fettling to get right, but their performance is quite incredible compared to rim brakes. But, if you have a Stoater, you'll already know that.
Good luck.
Simon.