Wading in here ...
Trek, Specialized, Cannondale
I've only been to a few shops so far. I've mostly been plodding through the internet. So. Many. Brands.
Several have mentioned demo'ing, but not IMO with sufficient emphasis. DEMO. I don't think bikes are like skis - even as a novice mountain biker, I think you will develop some opinions. You probably won't know why you like one bike better than another, but I bet you will find that some suit you better than others.
Last time I bought a bike, I demo'd several very popular ones. One that was great "on paper" was just awful for me on the trail. But other people like the bike. You don't know what you want. You don't know the jargon (geometry, head tube angle, 27.5 vs 27.5+ vs 29) ... and that's fine. Just ride a few bikes and decide which one you like best. If possible, ride the same trail with each one. As others have mentioned, there are demo events that are free, and there are shops where you pay to demo. Usually if you pay to demo, that shop will deduct the cost of any demos (of THEIR bikes, of course) from the price of the bike you buy.
and knowing
@Andy Mink he will never need what a $5K bike will bring.
Wait, are we talking NEED or WANT? I was unaware anyone needs a mountain bike at all ...
It turns out my (wife's) budget is significantly less than anticipated. Like zero. There will have to be some negotiations. It could be harder than peace in the Middle East but I'm committed!
My husband and I used to fight a lot about the bikes and skis I wanted to buy. We settled on an alternate approach - I withdrew $X into a "bikes and skis" account every month. I didn't have to ask him when I bought bikes or skis. He didn't have to be the fun police. We also agreed that if I sold any of our household clutter, I could apply the proceeds to that account. It worked pretty well, and I didn't have to deal with his "feast or famine" emotions about finance (we all have some weird emotions around finance, I think).
I mean its pointless to have any money when you die. Just spend it, or give it to your kids if you have any....not that death is anytime soon but when you die with 5,6 or 7 figures in your bank account, its kinda of like who cares.
Except that you may live longer than you expect, medical bills can go to six figures, and the current state of the market can really affect you when you're relying on it for income (while you're inherently losing money via inflation if you stick it in a mattress/savings account). Also, your kids will spend a lot less in taxes if they inherit your money than if you gift it while alive (depending on amounts, but I don't think most of us are in the "have to worry about inheritance tax" category).