Building one yourself will save you a lot of money. Plus the joy of "Hey I built that".
Fun can still be had on a cheap locally bought mountain bike, Craigslist, local paper etc, this time of year decent bicycles can be picked up for under $50. Bolt on a $150 kit and away you go

Many sponsors on the sidebars/top/bottom have lowered their prices for the winter months too. So it's a great time to get into this addiction *cough* I mean hobby
Stock kits usually come with a 44 tooth rear sprocket(I've had one with a 42), and that makes for a good overall power range. You can always change it out for a larger sprocket for dirt trail climbing torque if you feel the need to.
To put a kit on a bike, you really only need basic tools and the installation manual covers everything. Its not very difficult. Plus if you run into any problems you can ask away, there's always someone around willing to help out.
Welcome to the forum.