Expect to spend about three grand for a good full squish mountain bike. I have a Rocky Mountain and I would highly recommend the brand. They are one of the best values in terms of features, weight, and quality. There are even some really good direct-to-consumer brands now like YT. For mountain biking you definitely want disc brakes, a dropper post, and around 120mm-130mm of front travel with a total weight less then 28lbs. Global Mountain biking network on YouTube is a great resource as well as local riding clubs and bike shops. Good luck!
The major choice to make, irrespective of brand, is what type of bike — which is dictated by the terrain you have available and would like to ride. There are subcategories, but a useful rough split would be: a cross-country (XC) bike for good pedaling performance both uphill and on the flat, but not so capable going downhill and in the rough stuff; an enduro if you want to ride rougher downhill terrain (roots, rock gardens, etc.), but still have a bike that pedals well enough up technical climbs and fire roads; and a downhill bike if you want to ride the roughest, most technical terrain, with the steepest descents, and you have access to lift-served mountains (not recommended to start with ;).
FWIW, I’ve had a such a great time riding the Canyon Torque this summer. It’s a long-travel enduro, really capable on the descents, and perfectly fine to get back to the top.
As for motivation, I would echo the sibling comment. Time in nature, good exercise, a ton of techniques to learn and master, a really sociable scene, such a rush riding tough and steep stuff. You won’t regret it.
[0]: https://youtu.be/EX1Kb6TEyn8 [1]: https://youtu.be/Y_xiq2mczX4A
Giant is a good value brand. They make a lot of components in house, so you get cheaper, "good enough" handlebars etc.
In that brand, I'd recommend a used trance 29 or a new trance x 29, depending on your budget. I'm happy to talk about bikes for days!
Trek, Norco, Rocky mountain are also good value. Just about any modern trail bike is excellent.