There are some pros to rotomolded plastic as a first kayak (or beyond). Cheap, rugged, etc. 2 big cons to plastic are weight (heaviest type of material) and even though they are hard to break, they are even harder to fix after you break one. The big expeditions away from civilization always use composite (fiberglass, kevlar, or carbon) boats because they are repairable. This all said, all of my boats are plastic.
If by a trip on the Green River, you mean the stretch where they hold the Green River race, the boat required for that would be very different for Maine Island Trail or San Juans. For MIT and SJ, you want a sea kayak or touring kayak (these are different names for same thing). Probably 16 to 18 feet long and 22-24" wide. Can have a rudder or skeg. Some light packers, especially if they are doing surf or rock play on the coast, may use 14-16' long boats.
The Green River is white water (advanced white water at that), and as such uses white water kayaks. For overnight trips, the white water kayaks are long for whitewater kayaks, but still very short as compared to sea kayaks. Usually 9 to 11 feet long, and always plastic. As you can imagine, how much you can carry is very different.
Skills needed for green river type trip versus sea kayaking trip are also very different. For MIT or SJ, basic deep water rescue skills, efficient strokes, how to navigate, and tides and currents (last is more for SJs than MIT). For green river, efficient paddling, rolls, dealing with major rapids, etc. are needed.
Which shop on the central coast are you going to? Central Coast Kayaks in Shell Beach? Monterey Bay Kayaks? None of the ones on the central coast really do much sea kayaking any more. They all might have a limited selection of plastic boats, and few (if any) composite boats. Last time I was at Central Coast Kayak, the longest sea kayak they had was the Jackson Journey at 14'. You might need to make a pilgrimage to California Canoe and Kayak in Bay Area to get a better selections. You can definitely do intro to kayaking with your local shop, but more advanced skills you might need to also look elsewhere.