1. Find out the maximum width of tires and weight your bus bike racks will accept. I ended up googling the tech specs for Sportworks which is what my buses use (probably yours too), and the narrowest one is the Sportworks DL2 which has a 55lb weight limit and 2.3in max tire width.
2. Check if your bike meets requirements, including whether or not it will fit in the mounting system. In bike racks with front hook mounting system which goes over the top of the front tire, you'll have to remove the front fender. The Lectric XP 2 with the battery removed and skinnier tires and rims just about clocks in at 55 lb.
3. Decide on a tire width and rim that fits your requirements. At the minimum I would look at rims which are double walled (strong enough for ebikes), 36 holes, with offset rim holes (to accommodate the increased spoke angle from the hub motor) and a tire-rim width combination that is recommended by the manufacturer (I used Schwalbe's tire width and rim width table located here). Good cheap rims would be the SE Bikes J24SG (which I used the 406x24) and the Weinmann DM30. Ideally the rim would also have angled rim holes but I had a hard time finding ones that didn't break the bank.
4. Decide on spoke cross pattern (I used 1-cross, but I think you can get away with radial on the rear hub, which would reduce the spoke angle). Measure for spokes (I posted the measurements of the front and rear hubs here). Input them into a calculator to get correct spoke lengths (I used Freespoke). The lower the flange exit angle, the easier it will be to mount the spokes. I did end up with an exit angle of 31 degrees on the rear hub, which I was just about able to mount on the J24SG rims.
5. Decide on spokes and tires. I would recommend tires certified for ebikes (Schwalbe has ECE-R75 and ebike 50 km/h certification on their tires, Grin recommends 13-14G spokes).
6. Mount your spokes, tires and rims and enjoy! I liked Grin's single-cross wheel build tutorial and Park Tools' wheel truing tutorial