The way I did it was to buy a reasonably priced bike, a 1000 watt Bafang mid drive motor kit and finally a battery (couldn't afford to buy it all in one go). Relatively easy to put together and as for restrictions, I've yet to find any. Let's say that I might get a speeding ticket in a 30mph zone, (if I was to ever go that fast of course ). The bike was £600, the Bafang Motor kit £610 and the battery £300 from Germany, so a nice bike for under £2k. Just to add, I opted for the battery fitted on a rear rack as I wanted the bike to look like a normal bike and not an ebike. At a glance you wouldn't think it's an ebike. Handling with the battery on back is just fine despite some saying it puts the bike out of balance, it doesn't with a mid drive.