Second hand bikes tend to accentuate the ones with electrical problems, or the just plain junk. Beware of bikes sold without a battery removal key (if required) or a charger. These are often stolen.
You haven't stated how much elevation change, how heavy you are, or what size you are. Bikes are like clothing, the wrong size is useless. You should be able to sit comfortably with your legs almost straight with the pedal at the bottom position. You should be able to reach the ground stopped with the tips of your toes. Your favorite posture matters what kind of bike you buy. People that ride with a flat back, hands on the drop bars, hips in the air, those ride the fastest with the least wind resistance. Ergonomics textbooks suggest the exagurated flex of the neck to see the road can cause spinal problems. Those of us still riding in our seventh or eighth decade tend to ride with a straight back and neck. Medium forward lean comes with mountain & gravel bike frames.
Elevation rise of >333 m in an hour indicates a mid drive is required. These are more expensive, but tend to come from companies with a good reliability record. Bosch Shiimano Yamaha drives dominate that market. The replacement batteries tend to cost more than L1000. A few minority brands install the bafang mid, that can be bought in some countries with a throttle. These tend to use generic batteries like the dolphin.
Flatter terrain allows use of the hub motor. If you do not start and stop often and like high speed, the direct drive motor is useful. They are sluggish up to 20 km/hr, then really come on. That linked new bike appears to be a direct drive motor bike. If you do start and stop frequently, geared hub motors have much more acceleration at lower speeds and climb hills at lower speeds a lot better. I ride a 1000 w Mac12t geared hub tuned for climbing multiple short hills with significant weight. I carry supplies to my summer camp.
Rough road conditions can require a front suspension or even front and rear suspension. These add L1000 to a bike price. Some people substitute fat tires, >3", but the low pressure required cuts range radically for a given battery size and eliminates the possibility of riding the bike unpowered. The best drive to ride unpowered is the geared hub, although bafang yamaha and shimano steps mid-drives do not drag the motor with your feet power off.
Majority brands with real alloy spokes, rims, cables, etc tend to be trek, shimano, giant yamaha cannondale kona. In the UK lots of raleigh stock. In the under L800 category you buy a lot of lead and copper in your "steel" and zinc in the "aluminum".
When you decide what kind and size of frame you need, check the categories threads for the best choices people recommend.
https://forums.electricbikereview.com/categories/discussion-by-type.174/
One way to narrow your choices is to visit local bike shops and see what they stock. If you require help from a bike shop, say for an electrical problem, or to true a wheel or set up a derailleur, most shops will not touch an electric bike they did not sell.
One way to save lots of money is to buy a quality used major brand bike, then install a hub motor. I did that. It requires ability to match up the colors of the wires, perhaps wire termination crimp tool, and perhaps a drill, vise, hand tools and stock of stainless steel fasteners. See diy kits thread for more information. h
ttps://forums.electricbikereview.com/forum/diy/
Use safety glasses when using power tools.
When I have electrical problems I buy substitute parts and throw away the part that is associated with the problem. I went through 2 junk batteries before I spent the $630 necessary to buy a good one.
High speed comes with attendant risk of injury. Before I was able to find a vented helmet with a chin guard, I broke my chin hitting gravel at 25 mph. You can not always avoid penalties of riding over gravel, ridges, ruts, potholes, high block separators, tree limbs or trash. Besides my Fox Rampage helmet I wear long sleeve long pant mechanic's wear of polyester/cotton, closed shoes, polyester gloves, to prevent road burn when I hit. Over 25 mph people wear plastic armor sold for motorcyclists. I wear safety glasses to protect from gravel thrown by trucks, and bugs in the eye.
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