Focus

Unlike bikes & steppers (manual devices), treadmills (electric devices) move you (just keep up). This satisfy the mind jitters. Its extremely valuable for ADHD.

Energy

Moving keeps blood and endorphins pumping. It keeps you alert and on task all day. Oxygen and endorphins help not just with energy, but focus. You'll need less caffeine.

Health

Calories and heart-rate. At my best, I've clocked 320 active zone minutes (Fitbit) in a day. That's 5.3 hrs of gym time. This eliminates the gym, saving time and money. At my worst, I clock the minimum-recommend 10k steps. Further, your posture is ideal while walking, better than sitting and standing.

Get extended warranty

Some mills offer one through their website. Amazon offers Asurion. With the budget treadmills, since YYMV and the reviews are fairly inconsistent, you'll want the peace of mind. I estimate the lifespan of them to be 2 years, based on my own experience.

Budget

The budget mills can be more loud, and deal less effectively with heat. To deal with this, reduce the amount of at-one-time walking (eg 30-45 min on, 2-5 min break). Generally expect to get around 2 years out of these mills; compared to the non-budgets which could last a life-time. Personally I take the trade-off; I don't know where I'll be in 2 years, and I also have an extended warranty. You can buy a new budget-mill every 2 years for 12 years before you hit the cost of the non-budgets.

Non-budget

These can bear more weight than the budget picks, and can run continuously for much longer. They're quieter, and they're likely to need less servicing. However, I can't in good faith recommend these, given I haven't needed to service my budget mills after 2 years; I contend that the budget picks are less an issue of quality; and more an issue of using them wisely, like non-continuous use.

Using the table below

TL;DR: sort by Score, enter a Price max.

Score is a weighted sum of each rows' attributes. "Weighted" because some attributes are more important than others. Eg, App support has weight=1, where Sturdy has weight=10. You can see how each cell ranks in the bottom corner of that cell, eg 10

Anywhere there's a dotted underline, click it for details. I try to make these count.

Lubrication

Adjusting the belt

Walk 30-45 min, break 1-5 min

Treadmill Mat

Adds a layer of shock absorption for your knees, and prevents shock damage to hard floor. Over time the rubber stoppers will at worst damage the floor, at best smear rubber that's hard to remove. Adds a protection layer against high-pile carpet, so you're not pulling debris into the hardware.

Standing Desk: Flexispot ($100-200)

My budget pick. I've abused this cheapie for 3 years without a hitch, so I don't see the point in the $1,000 Herman Millers championed on https://www.reddit.com/r/StandingDesks/. But if you want to splurge, browse that subreddit. I have seen Flexispot starting to gain traction there recently; in particular the E7 / E7-Pro. You'll definitely want an electric desk which can sit or stand, because after walking or standing all day, you'll need a break sometimes.

Shoes: Brooks or Hoka (Men, Women)

There are shoes more tailored towards prolonged walking or standing rather than running. I research what nurses swear by (articles, Reddit, etc) since it's the closest lifestyle to a walking desk. They're quite bullish on two: Brooks Ghost Max 2, and Hoka Clifton / Bondi

Ergonomic keyboard & mouse

When you're walking or stepping, you're moving your arms more than usual. People often develop RSI (Repetitive Stress Injury, a cousin of Carpal Tunnel) when seated with a standard mouse, due to the arm motion - that type of wrist motion is bad for you. This is amplified big-time on fitness desks, due to the fitness motion. It could take 6 months, it could take a 5 years, but almost everyone I've talked to who have fitness desks have experienced an RSI uptick. Ergo peripherals solve this definitively.