Here's the thing; if you have a base bike that you're looking to convert and a fair amount of know-how you can build something that'll get you 40-50 miles on a charge with a lot of power for around $2k (or a bit less).
If you go with something like the
Dawes Haymaker 1500 (which is a solid aluminum frame, full Deore/Deore SLX human-powered drivetrain,Rockshox Dart 2 (or XC28) front forks, and is currently retailing for $300 on sale), then you have about $1700 left to play with to put together the electric part. If you get one of the 48V 1000W YescomUSA kits off eBay for something like $300-$500. You can upgrade the controller that comes with it to a
72V/40A controller for peak power somewhere on the order of 3kW for another $175, leaving you about $1000-1200 to spend on batteries. If you want to get 40-50 miles of range out of that setup (which shouldn't draw more than 40Wh/mi or so) then you're going to want 2-2.5kWh of batteries to work with. 20S LiPo will give you 74V nominal, and if you put together a 30-40Ah pack then you'll have 2-3kWh of battery available, which should last you a while. If you want to get 2-3kWh of 20S LiPo for cheap, then look into the 4S 5Ah Turnigy hardcase packs on Hobbyking. Each of those packs retails for about $25 plus shipping, and 30 of them arranged as 5S6P will give you 2.2kWh of battery (74V, 30Ah) for about $800. Add in a 20S-24S BMS from BMSbattery or AliExpress, and a frame bag to hold the batteries and controller, and you'll come in right around the $2000 mark.
The bike you'll have when you're done would be capable of 30-35mph unassisted on the flats, and having the weight of the battery pack and controller (which will come to around 40lbs or so) in the front triangle rather than hanging off the rear rack should give you a lot better handling. The electrical system would be capable of delivering four or five times more power than the one on the Prodeco (2-3kW on the homebuilt bike, 500-720W on the Prodeco), and would have about four times more battery (16Ah at 38.4V gives about 615Wh on the Prodeco, as opposed to 30Ah at 74V (which gives about 2250Wh) on the homebuilt bike). The main drawback to doing that is weight; the battery and controller plus assorted wiring will likely weigh about 35lbs, and the hubmotor will probably weigh another 10 or so. Add on the weight of the bike itself (30lbs or so) and you'll finish up with a 70-75lb system, as opposed to the 59lbs of the Prodeco. You're not going to feel the extra ten pounds much when you're riding because of the increased power, but it might be a bit much if you need to store the bike somewhere besides a first-floor room or garage.