That's the old Currie Electrodrive motor on that bike, a design that's been around for years. It's fairly powerful on an alloy bike but a bit noisy. They've had the occasional motor problems but as you say, spares are freely available.
At £110 for a spare battery it's a 24 volt lead acid SLA type and I don't think an SLA bike with that old type motor is a particularly good buy these days. Having said that though, it can do the job and there's not much choice at this sort of price.
However, check out bikes like the 36 volt Synergie Mistral, alloy frame, lithium battery and hub motor in a similar price area, to my mind a much better buy.
Synergie Mistral
Synergie Mistral on ebay
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