Without a counter shaft for the gears, and differential unit, a rear wheel driven e trike will be a pure dog to ride. Both those you link to are conversions using the same kit. If you wonder why they both have little use it's because they are pigs to ride and potentially dangerous. Stick to front hub drive on a trike but watch your cornering as it will pitch you off in an instant with a tad too much power on. Keep a close eye out for adverse camber on roads & tracks and stay well away from dropped edges where Tarmac meets grass as either will have you on your side with a leg trapped under a crank.
A nasty trike will end up rusting in the shed from where most seem to emerge.
Look again at the link I gave you ,these units are on sprung flexing arms which allow you to lean into corners, you can set them a shade high and ride normally with the support wheels just off the road, when coming to rest lean on either side for support. An elastomer steering damper will improve the overall feel when cruising along. Safety on mounting/ dismounting can be assured with a latching brake. Using these the cost should be less than £200 and you keep your bike then switch them too another bike if upgrading.