Welcome to the site.
Unless you are a maintenance wizard, it is best to buy a bike from a dealer that will back it up with service. I'm pretty good at maintenance. I ordered some kit parts from the internet to convert the bike left and wasted $500 throwing things away or abandoning them for something better.  Cheaper than a whole bike, though. Only one dealer is within 100 miles here with one expensive brand,  and I had my project going before he started up. 
If you're in Chicago, you're in luck.  There are dealers there. Go down to Mike's e-bikes who posts here a lot, or Crazy Larry up in Wisconsen. Big selection both places. Other stores may be nice but don't get talked about here.  Try some bikes on for size, maybe test ride a few. Take your helmet!  For lowest price point at decent performance look up the magnum & juiced dealer.  Not a lot of hills around Chicago. 
Have fun shopping.