Don't fall for it. At least 5 mph if you or family members are only going to walk. Up to 8-10 mph if anyone is going to run, or ever plans to. Increments of 0.1 mph. Start speed of no more than 0.5 mph for safety.
Gradual starts are divinely safe, compared to a jerk and go.
At least one year manufacturer's warranty on parts and labor. Incline adjustments that require you to dismount and move pins or other mounts on a support leg. Shaking, rattling, or squeaking while you workout. If those prices are too high, then consider a bike or stepper instead. A tall person or someone with longer legs will need a longer surface.