I have a Poseidon X, and it's an amzing value (if you have a friend that can assembly it). But that's a gravel bike. I can't vouch for the mountain bike offerings.
Hydration, map if available, first aid supplies (incl. triple antibiotic and the kind of gauze you can stuff in an "impalement" type wound), hydration, a light (if it gets dark on you), multitool, something to fix tire issues depending what kind of tires you have (levers, inflator, etc.), hydration, and maybe snacks. Don't underestimate the need for a light. If you start later in the day and have equipment problems needing fixed, being on a trail without a light can "greatly extend" your trip.
Trailforks trail app was mentioned on the other thread. If you have the "pro" version on your phone, it will show you where you are on the trail & which direction you are headed, which can be a lifesaver in unfamiliar areas.
Oh yeah, bug spray if it's that kind of area and time of year.
Bikes are a worm-hole and is really dictated by type of terrain. 20~30 miles in one go is serious mileage if the terrain has any degree of technicality to it. A good, basic, entry-level, trail-worthy hardtail like the Giant Talon series can be had from several manufacturers in the 700~800 range, at the bottom level of component spec. Full suspension adds about a grand to that, again with the component spec ranges incrementing up from there. Nicer component fit-outs move you upward into the 3500~4000 range, at which point the full-carbon frames take over and push the prices out of sight. And I do mean "stupidly" out of sight.
If you're on a budget, the higher component sets are really not worth the price on a 32-pound bike, unless you're racing (just my opinion).
Don't underestimate the importance of tires. Full suspension is darn nice if the trail is technical (especially sketchy downhills), but if it's a lot of flat trekking, hardtails are fine. Tubeless tires that you can run down into the 25psi range add traction and a "bit" of suspension, and good tubeless rubber significantly upgrades the effectiveness of a hardtail bike. As long as you don't run them so low that you roll the rubber off the rim, I have found them to be a lot less prone to flats than tubed tires, because it eliminates "tube pinch-flats" which are the majority of bike flats. You would either have to completely pierce the tire, or roll it off the rim at too low pressure.
Right now the new bike market is cold for the industry. Values are easily found online.
Finding what you want in a bike shop is possible, but some sales persons might try to upsell you something that they want to unload.
If I were starting out I'd get a fat bike that has 27.5" rims and purchase a compatible set of 29er+ wheels to put on for faster rolling. Switching them back and forth is pretty easy and gives it versatility. If you build a set of wheels, the hubs MUST be compatible. I almost made that mistake. https://www.the-house.com/FRAKLTDMG...UX2tYaxtO5kFMdo2XfRoCwuIQAvD_BwE&gclsrc=aw.ds
I have nothing to add to this other than I have a trek, I like it. I have never taken it off road though.
I felt the shop off Pendleton pike in Lawrence (Matthew's?) Was pretty honest and not pushy when we bought our bikes that have hung in the garage for a few years now.