Sounds like you are full of bad luck lol I've had many bikes before from 80 to £190 max. Never have I had a problem with any of them before. And I ride like mad when I RIDE so I dont know how any of them actually survived? They all had to go due to "old age" no other reason. Apart from the £190 folder bike I had which my cousin managed to break the crank off of.....who knows how it happened. But yeah.
Not so sure it's merely bad luck my bike had exactly the same brakes and they catastrophically failed!
As far as e-bikes go I have to say from experience you need to build your own to get the best from your money, because otherwise you will be constantly repairing the cheap pre-built one.
A decent road bike fitted with a 250W crank drive motor will do 25mph assisted no problem, I would highly recommend decent front and rear hydraulic disk brakes though because of the momentum of such a heavy and fast vehicle, and fat tyres to reduce the effect of bumps and road surface defects.
And make sure that the battery has actually been independently tested to be the capacity it claims to be, so many batteries vastly overstate their capacity.
Lastly, a throttle is invaluable if not essential, you can be pretty screwed without one if you have to start climbing a hill mid-way in a high gear!
Generally I would say £600-£800 for the base bike, another £150 on things like brakes and decent lights, and then the e-conversion kit on top, for a top of the range legal e-bike.
Look, we're clearly not going to agree on this (even though I'm right) so we'll just agree to disagree and if you think a bike with a "derestrict" button is legal then good luck to you.
That's pretty arrogant given there's no case law to suggest it's illegal and it's not explicitly stated that the speed limit cannot be adjusted by the computer. But whatever, it's never going to come up in court.