I used to run salmon charters on Lake Michigan. You are in Seattle, I assume salmon are your target. Speed will be a problem. Again I assume you have a single engine with a 26+ wheel. In gear, you will probably go 3-4 mph. Too fast for salmon in our area. We go 1.5 to 2.5 mph. You can add a trolling valve to the trans which works well for some.
As far as downriggers, big Jon is the only way to go. 90% of the charter boats around here use them.
www.bigjon.com
Manual riggers get real old real quick IMO. Especially if you are yelling Fish On a lot