One of the biggest problems with buying a complete bike is the steerer tubes are often cut flush with the stem and insufficient length remains to adjust stem height.
Fight back! If you are buying a complete bike, insist on a long steerer, an inch above and an inch below the stem. Yes, this seems excessive, but some folks need a higher stem than others and it's certainly cheaper than buying a second set of bars (riser). You can always cut the steerer later.
Bar width at some point in a biker's "career" is going to become standardized, so for example I have come to prefer a 750mm bar width. The bar width and bar height I prefer are based on riding with a preferred ETT, in a sense these measurements are my comfort zone. All my bikes have the same ETT within 5mm, this is not a coincidence.
The one thing that varies across my bikes is reach, but reach really only matters when you are out of the saddle. Longer reach for downhill oriented bikes. Shorter reach for XC oriented bikes.
Things to consider:
Bar position: high, low, far, close, and sweep angle/rotation.
Seat position: high, low, far, close, and angle. Don't set your seat too high, the "roadie mentality" need not apply to mountain biking. Keep it comfortable.
Controls position: Roll those controls back so they rest comfortablely in your hands without having to move your hands. Too many people run their brakes angled to far down, so in the seat riding requires too much finger reach to operate the brakes.
Age: Yes, as we get older we can get uncomfortable being in an overly low or overly stretched out position. I found that raising my bars made a big difference. I also really appreciate a steep STA on my bikes with a longer reach.