Ha, just realised im answering you in 2 threads..
Ebikes are very modular, there is the bike, the motor, the battery and the control system thats it.
there are 2 options when it comes to motors, Hub/wheel drives, or mid drives, hub motors are literally fitted into replacement wheels, mid drive motors drive the crank you pedal and use the gears.
Hub drives are fit n forget, Mid drives can involve more maintenance/repair and will wear chains and gears etc rapidly compared to pedal power. But if demanding more from a bike to climb steep hills, pull cargo, or go really fast, the mechanical advantage of using the bikes gears are a huge benefit.
Hub motors are cheap and at and under the speed limit have plenty of oomph on the flat and in my case (fat ol bloke) will pull me up the steepest local city hills with some help from me comfortably sat in the saddle at circa 8mph at the crown of the longest and 12mph at the crown of the not so long..
neither at very long btw,... but one i would attempt in 1st gear puffin n panting out of the seat , the other forget it i would walk/push and still feel it pre conversion.
Bikewise, something comfortable to ride with good brakes Some suspension is good as you could spend more time in the saddle with less need to stand on pedals to move them with a motor.. And if doing a diy conversion, with a suitable spot for the battery, mounting high is 'bad' or less good as it raises the bikes centre of gravity. a full suspension mtb may not have room in the frame for a battery for example.
Seatpost suspension is a thing, trust me its worth considering..
Battery - depends on range requirements, obviously bigger is better but this is the most expensive component... And never buy uber-cheap!
Control systems, the more generic the better, the more flashy and anything with battery communication is to be avoided imho. Some manufacturers use the control system to their advantage such as prohibiting use of a generic cheap battery (thats where battery communication comes in) ....
I bought and fitted a yose-power rear wheel kit all inclusive with battery charger and tools for circa £400 last summer and 1 year on i still get a lil grin when on it..
For an off the shelf option afaik both wisper and woosh (uk suppliers) provide bikes with expected longevity.
Argos sales are worth checking some cracking bargains have been there recently like a bike that would suit you for £360.. ( 600 on ebay now iirc)
