I started biking to work (and being car-free) 11 months ago. I've lost a lot of weight and am still 230 lbs with a commute just as you described (5 miles, uphill at the end) and can do it efortlessly now (not so effortlessy at the beginning, but doable still). I know you can work up to it if a big guy like me can! Also, you made it half way and back, essentially you did the whole one-way commute, so I think you can do it no problem!

Also, I started out on a 90's Eddie Fisher Mamba (an old mountain bike) that I just put smoothe tires on. I still use it sometimes as it's fun for jumping curbs, riding down stairs like a little kid, etc. I say just save some money for a bit, get some smoothe tires and ride the mountain bike for a while to give yourself some time to figure out what you want in a every day bike! I thought I might like a hybrid at first, but after reading and riding for a while I wound up with a touring bike (Raleigh Sojourn) that I'm extremely happy with and probably won't ever get rid of. (That new Surley LHT with disc brakes is pretty tempting, though...).