There are lots of ebike kits on ebay for under $250 from sellers like Yescomusa, Vpower, and xcceries. This is a fair bit cheaper than the kits respectable retailers sell. The ebay kits also come with extras like lights, racks, horns, and battery "fuel gauges." I could sum up this whole review in one sentence: "Junk destined for the trash can," but I'd be missing the crux of the issue; you're risking your safety by using cheap components on a motor vehicle. It's not the core performance of the kits is poor, but you'll see that for just a bit more you can get a really good kit.
The throttles with the fuel gauges on them are useless. They have two positions dependent on the voltage of your battery pack: "High and empty." The gauges go to "low" with only 10% of the battery left; not very useful at all. The throttles with the LED fuel gauges are also dangerous; they aren't water proof and they are known to get stuck in the on position when they get wet!
The racks and bags that come with the kits are under built for the task of carrying a heavy battery. Why any one would trust a flimsy rack they got for free with an expensive and potentially dangerous battery is beyond me. Some kits secure the bag to the the rack with velcro...which isn't enough so they throw in bungee cords.... seriously? It's like using duct tape to patch holes in a gas tank. Even if you could securely mount the battery to the rack the thin metal racks will bend and flex. This can cause handling problems.
Another huge problem with these kits is the undersized and low quality wiring and the connectors they use. Copper is expensive, so they scrimp and try to get away with the smallest wires they can. It's not unusual to see the wiring and connectors they use to get hot enough to melt the insulation! This heat represents wasted power; something in short supply on a battery powered vehicle.
If we plug 16 gauge wire into my I^2R power loss calculator @ a 25 amp draw, we see that these undersized wires will be wasting 80-30 watts of power!
The controllers are another problem with these kits. Usually they work fine, but they aren't the Infenion stuff we've been able to reverse engineer. So when you want features like regen, anti-theft, cruise control, adjustable LVC, cycle analyst compatibility, or the ability run them at different voltages, you'll be out of luck. These controllers are typically around $20-$30 shipped direct from China, and the reasons stated above are why people choose to pay 2x-3x as much for a controller from Lyen or ebikes.ca. It'll be just one more thing for you to upgrade in the future.
The only thing you won't need to upgrade is the motors. Typically, they're all very similar.
However, none include or even mention torque arms!
Hopefully I've convinced you that these kits aren't worth your time and money, but if not there is one last point to consider: these kits are dropped shipped from China by people that may or may not know what they're selling, care if it works, or even speak English. If you have trouble getting everything to work or something is defective, the best you can hope for is getting a refund after you pay for shipping back to China. Good luck, and check out my list of retailers that off better quality kits for just a little bit more.