Mid drives usually require a 68x113 bottom bracket so I'd make sure your Windsor has that, along with whatever kit you're looking at. A mid kit won't fit in an old Schwinn one-piece bb shell without a threaded insert.

An 8 mile commute only needs a 10ah or so battery at 36 or 48 volts. How steep are your hills? that bionix kit is only 24 volts and won't give much help up hills over 5% or so. For the price of the bionx you could get a 36 or 48 volt kit, much more power and more options to upgrade in the future. There are several stateside vendors in that price range, no need to pay for shipping from China, especially on your first ebike where you may need to return something or order spares. check out ebikekit.com, they have monthly specials, also a couple vendors in california in that price range.

One advantage to a front hub kit is you should be able to swap it between your current 2 bikes, as long as the forks are steel. A 350w 36volt 10ah system would be a good start. I have a 250 watt bafang mounted in an old flat Schwinn fork and it works fine but i'm not using a front brake, you'd need to make sure your brake caliper will hit a 700c rim. Maybe try the windsor wheel in the conti, see if the brakes will hit. You don't need disc brakes on an ebike as long as you keep the speed within the brake limits, although a cutoff switch hooked to the levers is a nice safety feature in case of emergency. $1000 kits usually come with that, good luck