An hour should be more than enough for 12 miles, traffic and lights allowing. If you’re of moderate fitness you’ll be able to do the round trip straight away, but a practice run on a weekend might well be a good idea.
Let’s see, 2 hours of moderate cycling a day will burn between 560 and 840 calories a day. If your weight is currently stable, and you do not eat anymore, you’ll lose around a pound a week. You will also be gaining muscle, so yes this would be a very effective way to lose inches and keep fit
OK, you’ve mentioned prices in pounds, so I’ll take a wild stab and say you live in the UK. You’ve identified your needs nicely, so forget spending £300. Purpose designed commuter bikes are ideal for your needs, and far cheaper than sports bikes. As an example, have a look at Edinburgh Bicycle’s ridiculously well priced Connection Everything you’ll need for commuting, and none of the things you don’t, all for under £150. I use one of their commute bikes for…well commuting strangely enough, and find it far better for riding around town than my vastly more pricey mountain bike. I wouldn’t bother with suspension at all - on roads it just means more weight for no gain.
Extra gear I’d recommend:
- Helmet. No argument.
- Good backpack, preferably with a drink bladder. You’ll want to change shirt when you get to work, and an hours cycling on a hot day is thirst work
- Bike computer, so you can log miles, calories and times. The wireless ones are a peace of piss to install and start at £30