Just a few things off the top of my head.
I'm not sure what can be had for $150 in the states, but if possible, go to a dedicated bike store. The quality is much higher and usually they have decent support plans (like free tune-ups, etc.). Trust me at department stores and places like Costco (or the States equivalent) usually sell crappy bikes.
You are on the right track with your tire thinking. Chunky mountain bike tires with treads are useful for traction off-road, but will create unneeded friction on a smoother road. Thin and smooth is your friend under ideal conditions, but you'll probably want something in-between. "Semi-slicks" would probably be good as they are smooth on the part that touches the road, but bumpy just outside of the smooth strip, which makes it safer on turns especially when it's wet outside.
Your budget probably won't allow you to replace the seat, but do not waste money on a gel seat cover. Better to get a more ergonomically shaped seat. As far as other accessories, you'll need a lock, but keep in mind when buying a bike that a dedicated thief will
always get your bike and no lock can prevent it, so don't go all out on your lock. Better to be more strategic about
where you lock it (my favourite tactic is to make sure there's a better bike around
). Also, I'm a big fan of bike-gloves, which decrease slipping in the rain. A bell is essential, but drivers usually can't hear them from inside the car
. Unfortunately your only option to get even with car horns is the super bitchin' air horn which requires a refillable air cartridge the size of a water bottle. It's about 5 times as expensive as a bell and I'm sure it's fun to scare people with but no one I know owns one, so stick with a bell. Make sure you get the ones that go *ping!* instead of *br-r-r-r-r-ing!*, they're much more manly.
As far as the actual bike, you are looking for a road bike or a hybrid. In your price range I suspect you'll be able to find a good hybrid. Don't rule out a reliable used seller either!