First off, welcome aboard -- Open Carry is a wonderful right, but like you mentioned, it is a tremendous responsibility. It is up to you to know and understand the laws and legalisms surrounding open carry in Washington State. Before you go full-bore OC, I implore you to keep the following things in mind:
1) Know and understand the law -- open carry is legal insomuch as you do not "warrant alarm" for the safety of others. Read RCW 9.41 for all firearm law in WA State.
2) There are caveats if you choose not to obtain a CPL -- car carry and carry around schools and school zones.
2) Know how you will respond to citizens and law enforcement. Most will leave you alone, a few will approach you with honest questions, and on rare occasion you may run into somebody who is openly hostile towards guns. When it comes to law enforcement, if they have questions, want to see your ID, or your weapon -- determine what response you want to take. Stand up for your rights, but do not argue or start a confrontation. Open Carry provides a wonderful means of ease and access in our own personal defense, but we are wide open to public scrutiny.
3) How you carry yourself is FAR more important than how you carry your weapon. If you are confident, friendly, offer a smile or converse with people -- ergo, act normally, people will be far more inclined to leave you alone.
4) Get a good holster and practice with it -- at home of course; practice weapon retention as well.
5) Knowledge is power -- read, ask questions, browse youtube; the more you know -- the more you UNDERSTAND, the better prepared you will be.
About choosing a firearm: not every gun is for every person. I recommend going to your local range (bulls eye in tacoma has a wide selection), practice with a few handguns you think may suit your fancy and go from there. In terms of price vs. your preferences -- Glocks and Springfield XD's will usually run from 5-600 bucks, though I do not recommend a Glock for a new shooter. The Ruger SR line has a good 40 cal that runs between 4-500 dollars. That is just for the firearm -- not including tax, ammo, and a holster.
All Federal buildings including national park buildings and post offices are no-go. Do not carry on school grounds, bars, courtrooms, or the secure areas of jails, airports, or mental facilities. Private property owners can ask you to leave at any time, if you do not, you are guilty of trespass though signage is only valid where you are already prohibited from carrying.