I assume your 50 mile range includes a decent amount of pedaling. I'd recommend something like a 700 watt-hour battery or bigger, but your mileage may vary. Watt-hours = volts * amp-hours. You could get away with less with harder pedaling. Going fast, lots of hills, and lots of starts and stops would shorten your range. Going slow with lots of pedaling would extend your range.
I'm most familiar with Pedegos, and they meet your requirements except for price. Our cheapest bike is $2295 MSRP, and you'd have to upgrade to the extended range battery for another $300, and I think you'd still have to pedal too much to get the range you need. It's doable, but most people who buy ebikes don't want to work that hard. The Pedegos with big enough batteries to comfortably do 50 miles with moderate pedaling run $3295 MSRP, well out of your price range. On the plus side, we have an extensive dealer nextwork and an excellent reputation for quality and customer service.
I wouldn't worry too much about a bike that requires more frequent spoke tensioning. Doing a rough spoke adjustment isn't too hard after you've done it a few times, and it's part of most bike tune-ups. Then again, it might mean the wheel would need to be trued more often, and that's more involved. Keeping proper spoke tension is more important if you carry more weight or ride on rougher roads.