Where do I start?

You want to build an electric engined bike? Cool!
I don't have much information for you, as I am no engineer. Are you looking to build your parts, or are you looking for an e-kit? I know a guy who would take the former route over the latter (as he loves tinkering on that deep of a level).

I would say check out the suppliers advertised here. I can't remember which ones specifically, but I remember seeing electric stuff on their webpages. Maybe one of them has a kit that would suit your desires/requirements?

I don't know much about the ebike kits. However, in any motorized situation you want to get the bike ready to handle having a motor and higher speeds. Brakes, axles/bearings, tires, lighting, ect. Most can slap a bike/ kit together, but being safe and making sure your ride is not going to break in a month is where you should start. Make sure your roller is setup how you like everything before you put the ebike kit on or any motor kit on for that matter.

Either way best of luck with your project

start by having your laptop by your build site and bookmark this web site,
lots of great help here,...!
have a grinder, dremel, drill bits, be ready to move and maybe reshape or weld some metal,...