I lived in Portland for 14 years. It's a great place to commute by bike, as long as you're on the east or west sides. South of the Sellwood bridge, or north of the 405 bridge, things get a bit dicier.

A few things I'd look for in a commuting bike:

1. Disc brakes
2. Room for wider tires (at least up to 40mm)
3. Eyelets for rear rack and fenders (front and back)
4. Avoid low spoke wheels (under 28 per wheel) if you can
5. Wide range of gears

Some accessories I would look into are:

1. Fenders
2. Rear rack
3. Bike bag or backpack
4. Front and rear lights
5. Reflective vest or jacket
6. Good rain gear

General advice about bike buying:

1. Don't buy something cheap just to fit a price point. Otherwise, you'll find yourself wanting to upgrade it later. Avoid upgraditis by buying what you want the first time.