I don't think that kit you show can use a display. It uses a thumb throttle and has no pedal assist sensor. All you do is power on, twist and go. It should have some LED's to show green/yellow/red as an indicator of battery power. That's enough.
Good thing you didn't throw money at an SW900. It wouldn't ever work.
Buy a inexpensive bike computer. It's actually a speedometer/odometer with a clock. Put in the right tire circumference and it's more accurate than any display. You might need to look hard to find one that 90 year old eyes can see. A display will show you this, sometimes the temperature, and sometimes the battery voltage, but sometimes they don't.
Pretty cheap single wall rim. I suggest, if you didn't already know, using rim tape. It will prevent the ends of the spokes from cutting the inner tube. Electrical tape will suffice.
Most of the sellers don't have much knowledge of what they sell. They are, on the other hand, smart enough to order a container full of bike kits and make big bucks selling them. Good for them!
