We've been through the environmental impact of ebikes v bikes on this/another thread. Turns out to be nuanced and not obvious. But essentially human + more food + bike has a greater environmental impact than human + food + ebike (yes, you will on average eat more riding a normal bike because you are expending more energy - and electricity turns out to be more efficient at propelling humans than human power). The additional environmental impact of the production of a new ebike is essentially the same as a couple of large steaks. That should not be a surprise as the additional components are roughly the equivalent of a cordless drill plus a couple of batteries. Compared to a car, moped or whatever it's peanuts.
I have never paid more than £500 for an ebike - either conversion kit for an existing bike or full ebike. Here's the fiido D3 pro, for example:
Fiido D3 Pro: Compact and portable, budget-friendly, powerful 250W motor, 37.2-mile long range, the top choice for city commuting and leisure riding!
uk.fiido.com
A perfectly capable little ebike that does at least 15 miles on one charge over my very hilly commute. It'll also happily carry me (80kg) and 20kg of shopping up a 14% hill. Customer service is a crap shoot, but if you are happy fettling as I am then it's a good deal.
Of course, if funds are limited or you want/need the exercise - a second hand push bike is always a good choice for commuting (and environment). I would probably still be using one myself were it not for the surgery, physically demanding job and hilly commute. But our household is carless and the rural area we live in poorly served by public transport so I'm not going to lose any sleep over using a ebike.