i started out with a Carrera from
Halfords. that would be in your price range. There is a lot of bike snobbery about Halfords but Carrera are ok (stay away from the apollo stuff) and it's more to do with the monkeys that work behind the desk.
if you're unfit and thinking about a 17 mile commute each way (which is what i do) i would suggest this.
Leave what you can at work and carry the least possible. Do you have shower's there? you will probably need one. Can work supply a locker for you to put your clothes in?
if you can leave stuff at work, a road bike would be better than a tourer because it's lighter.
Is there an alternative way of getting home to begin with, ie. train? it's quite a jaunt when you haven't done it before (the first time i did it, i was as unfit as you and nearly died by the time i got 5 mins away... i thought "this is a bad idea!") but i did do it and two weeks later i was thinking this is a breeze (well, not quite but i managed it a lot better) but i've never smoked either. you might find your really knackered if you do round trips to begin with, so a good way would be to either cycle to work and take the bike on the train home, or just do it a couple of times a week to begin with.
working up to 3 times a week round trip to begin with will be sufficient to substantially increase your fitness level. don't ride too fast or your energy levels will drop during the afternoon and you'll be sleeping at your desk!
then when you get as fit as some of the boys on here you can do it 4 or 5 times a week and have buns of steel.
the first time is hard, but it will only ever be easier after that. even when i haven't been on my bike for a few months i slip back into it really easily and i've never felt like i did the first time i did it (and spent all day wondering how i was gonna get back up the really big hill i had freewheeled down!) (i didn't, i had to get off and walk but i've made it up everytime since)