[snellemin]

Pre-build stuff is pretty good if you don't mind the price. Right now mid drives are all the rage between the good manufacturers. There is only one store in Houston where you can buy a good pre-build bike. Some other states have zero places where you can go check one out. But if you can find one place near you, I suggest you go check it out and see what system you like. Then either buy from them or build something similar.

No bike manufacturer offers a bike with 24s Lipo capability. So you are kinda stuck putting a system together yourself. For your requirements, I would buy a Magic Pie II motor wheel setup. It's thin, supports both rim + disc brake rotor and has plenty of torque. It will do 1000W+ all day long. Climbing overpasses will be a piece of cake. Controller wise, I suggest you go with either a 12fet or 18fet 4110irf controller by Lyen. Lyen is on the Endless forum. Tell him the voltages you which to run and he'll put in the proper resistors value for you inside the controller.
I like Zombies program version for the controller vs what Lyen provides.
Get the Cycle Analyst V3, to keep an eye on things and messing around with cruise control, torque control, etc.
If you are going to use a hard tail bike, I suggest you go with nice fat tires and/or suspension seat post.

People on the other forum will tell you to go for the Crystalyte motors. Personally I am not impressed with the H35 series. The H40 series looks like a better product if you want to go with that manufacturer. Ebikessf.com carries the motors and I have purchased from them before.

If you don't want to got up to 90V, you can look at stuff from Bionx.

A 3 speed switch is nice to have and that can be used directly from the controller itself. Regen braking is also nice for saving your brakepads.