I'm not a pro on this matter but in my opinion it could go two ways. You might be greatly happy with the bargain fully ride ready ebike and accept its shortcomings in quality - or just be sorry you didn't spend a bit more on a 'proper' bike. I don't have any experience on one but I'm guessing it's not going to have brilliant motor performance or all that great qc on the bike either.
I was considering getting or building on a cheap bike like this at one time also, except with crank drive and the battery in or underneath the frame. I saw one pre built like that not so long ago and lost the link. It was a couple hundred more expensive than your link though (if anyone finds it, I wouldn't mind looking again)
In my opinion, the weight distribution makes a big difference to the ride feel and having all the weight of the motor/battery at the back will affect the handling negatively. Though on the plus side it
is a smallish battery, but it's still high and behind.
I've heard bikes like that have a relatively low weight tolerance so once you have the electrics on it there's not much play left for rider weight or luggage of any kind. It's bound to put extra stress on the frame, motor and battery
Last words then, those type frames can be picked up for around 110 I think shipped. As for the electrics I don't know exactly what's on there or how much it'd cost. But I can assume it's pretty low level and a bbs crank motor and 15ah battery would be about 645. So if you were willing to build one the cost to you would be similar to the average for a bike like this except with better gear on it. It's not neccesarily something you want the hassle of though I understand.
It's a cheap price though and may do you fine for all I know, and if you decide to get one I'd like to hear what you think about it