There are lots of people here who can advise you on suitable bikes in your budget - of which there are plenty - but I'd avoid Halfords if I were you. Better to buy from a local bike shop and if possible an ebike specialist.
As has already been mentioned, aside from the choice of bike, security should be your number one priority. If you haven't already done so investigate secure cycle storage. I work for a university and we have secure cycle storage for staff but it's hidden away and you wouldn't know about it unless you specifically asked the security/facilities department. Also look to see if the local council has any nearby secure cycle storage facilities. Above all else make sure your bike is insured and you use at least two very good quality locks (these will cost upwards of £100 for the pair) and lock the bike properly through frame and wheel to an immovable object (there are plenty of YouTube videos on this). You may be able to insure the bike on your home contents policy but check the small print. If not a separate policy will probably cost around £80 but it would be madness not to insure the bike in your situation. Before I knew that I had secure cycle storage available at my destination I was considering a covert GPS anti-theft device, there are several on the market now, I'm not sure how reliable they are but its something to consider if security is an issue e.g.,
http://www.spybike.com/ https://boomerangbike.com/how-it-works/. Also if you work in an office and you have the space then it might be worth considering a folding bike which you can keep in the office, Woosh do a full-size MTB style folding bike, the Zephyr-B which might fit your needs
http://wooshbikes.co.uk/?zephyr-b
I commute on my ebike and in terms of clothing you'll just need a rain jacket big enough to go over your regular jacket/coat, some waterproof over trousers, and some gloves. For winter I'd also recommend some padded/insulated over trousers and some thick gloves.
It goes without saying that you'll also need a helmet and lights - most ebikes come with lights, for commuting it's useful to have integrated lights which run directly from the main bike battery as you then don't have to worry about replacing batteries or recharching etc.
Finally I'd say, go for it, I've been commuting on my ebike for the past year and it's still the part of my day that I look forward to the most. Even the winter whether hasn't put me off.