I just want to congratulate you on deciding to bike to work in Seattle. This is probably the best time of the year to start commuting by bike around here. The weather isn’t to warm nor is it too cool, and you’ll have many riding days if you are limiting yourself to those without rain. Plus, there are many biking activities, outside of commuting, in and around Seattle.
Anyhoo, I really like my Trek520. It’s a touring bike, which looks like a 10 speed or road bike. The frame geometry is designed such that you have more space to stretch out your upper body. If you’re proportioned in a similar way to myself, slightly long body and short legs but not noticeably so, and you’re short or, as one of the bike salespersons I spoke with calls it, diminutive (5’6"), then you may want to look at touring bikes. I feel scrunched up on road bikes that fit people my size. Also, touring bikes come with tires that are a wee bit wider and have just a little bit more tread on them than your average road bike.
I want to second the Bikesmith recommendation. Although they may not have had what I was looking for, they really knew their stuff and made wisecracks about my “diminutive” size. Two other places you may want to look for used cycles are R&E Cycles on University Way near Lake Washington and Second Ascent in Ballard. Gregg’s Cycles near Green Lake, Ti Cycles near University Village and Free Range Cycles in Ballard sell new cycles and equipment. I’m uncertain, but Free Range may sell used bikes as well. Be sure to take the bikes you’re interested in for a test ride.
I would suggest buying some riding gloves, a comfy pair of stretchy pants, shirts made of a breathable fabric, lights, lights, lights, a rain jacket just in case, a helmet of course, and lube for your chain. Keeping your chain lubed will make your ride much easier. Plus, I don’t really like bicycle shorts. They make my ride much more uncomfortable. I wear a pair of cotton/lycra knickers, which suit me for any ride length. As for seats, that’s really up to your bum. Mine seat has a depressed center for one’s soft bits and two bumps on the back for your sitting bones to rest on. I think it’s a great seat, but my friend really likes her large, cushioned seat that she bought for her bike.
I prefer panniers to carry my load. I find that my bum becomes soar when I carry my stuff in a backpack. Perhaps bicycle shorts would benefit me then. Whatever you buy to carry your load, be sure it is waterproof. I know you don’t intend to bike in the rain, but it doesn’t hurt to be prepared for inclement weather.
And a link to the City of Seattle Bicycle Website if you want a free bicycle path/lanes map of the Seattle area.
Good luck in your search and enjoy the ride! 
Sorry about the length of this post. I just get so excited sometimes.