Trigger warning: opinions regarding "modern mountain bikes."
For $1400 I'd watch the local Craigslist and Facebook marketplace listings for a used hardtail from 2016ish or later. Something with 69 to 66 (ish) HTA. 29er/700 tubeless ready wheels (27.5/650 is OK too) at least 2.3" wide. 1x drive train (46T or more big cog in the rear, clutched derailleur). Dropper post. Wide (650mm or more) handlebars. Short (<60mm) stem. Disc brakes (these will probably end up being hydraulic because that's become standard, although IMO cable actuated is fine for most people). That would cover riding a lot of mountain bike trails/singletrack in and around AZ as well as bikepacking routes on forest and jeep roads, doubletrack, etc.
Modern mountain bikes are really good. The geo works extremely well with the longer top tubes, shorter stems and wider bars. Big wheels have amazing ability to roll over trail obstacles. With a little technique and proper setup these bikes are incredibly comfortable, safe and capable. These bikes also cost more than fully rigid mountain bikes, ATBs, hillibikes (sometimes), whatever you want to call them. They have suspension forks, dropper posts and possibly hydraulic brakes that need to be serviced and maintained (i.e. complicated)
A Surly Karate Monkey or Krampus both fit the bill and could potentially be had for under $1400. A Sklar or Crust is pretty unlikely. Kona Honzo or a Marin are also good bang for the buck. Here's a list of budget hardtails
The Radavist published recently. Any used model would be fine. I wouldn't be too worried about the brand, so long as it's a legitimate bike company and checks the boxes above. A mid tier Shimano or Sram group (or at least shifter/derailleur) wold be ideal. The new Microshift 1x drivetrains are cool too.
All that said, it may be worth honing in on what your brother means to do with the bike. I hate to say it but "trails and bikepacking" is pretty broad by today's standards, especially in AZ and the Four Corners region. Does he plan on riding rocky, steep, technical trails that require you to lift a front wheel or roll down obstacles? There can be lots of that in AZ. But if he has no intention of ever doing that, a fully rigid bike with biggish tires may fit the bill just fine!
Laying it all on the line,
John in Minnesota