Me and my buddy used to fish Rattlesnake lake all the time with his 1965 12' shallow v Sears aluminum boat and electric trolling motor with plenty of room left over for another to fish with us or room for the cooler.
One thing you might think about is looking and maybe spending a little bit more or just get lucky finding a deep v aluminum rather then a shallow v in case down the road you want to put a bigger motor on it and fish the Deception pass area up that way when the Salmon are running through. The Deep v will handle rougher water much better the a shallow v and only run a little more dough on average. That way if you are so inclined you can fish some of the safer salt water areas up that way.
i have a 12' v-bottom rowboat type thing and what i have found is it is kind of crowded for 2 people to flyfish simultaneously out of. it is also woblly if trying to stand up in, although this is not recommended anyway due to safety reasons. a deeper v may be different. i have fished out of a deeper v 14' boat and not had these concerns.
I have a 14 ft. fiberglass boat with a 15hp outboard that works fine for 2 people flyfishing and can easily fish 3-4 otherwise(as long as they aren't to heavy). The hull weights about 200lbs and the whole setup with the trailer is about 750lbs and it's pretty easy to move around and tow behind a little 4cyl car. A 12 - 14 ft boat is a lot lighter and easier to handle. Grandpas 13ft smokercraft makes my boat look like a snail with the same power on it.
I just bought a new lund 12 footer, with the trailer, figure at my old age can't hardly lift my 55 pound pram anymore, I can launch free at pass lake with my park pass. So if you see an old man with a cane come on down and help him launch. LOL I'll be able to tell some secrets of the lake.
i've got a 12' welded klamath and i fish it just fine with my 2 boys. it's 63" wide (only weighs 150 lbs dry) and i can stand up in it to cast alright. i'm also a fan of the deeper V aluminum boats...quite different stability-wise from my old 12' aluminum sears boat!! if you're not in a huge hurry, you can find a nice wide deeper V boat if you just keep your eyes open. i drove to the tri-cities for mine, but the guys gave me a screaming deal (and his dad threw in a nice galvanized trailer) . good luck and let us know what you end up getting.
Just go to craigslist.com and pick up one of the free boats or cheap boats with a hole in it. Go to the auto parts store, get some fiberstrand, bondo, or other fiberglass body filler. Mix it, patch it, and get out of the water. Also the electric motor is very nice. It is good to troll flies to the location you want to fish. Then fish. Also you can get a drift sock if you want to slowly drift. Also get some floatation seat cushions with some of those stadium seats. (crazy creek) Strap them to the cushions and you are set. Also get a 2 inch 2-3ft plastic tube. Cut it at an angle and you have yourself a portable relief (toilet) tube, which is better then leaning over the side. Carry sheer pins, pliers, and flush the motor if you use it in the salt. Also get a book on fishing boats for personal improvements.
I just bought a 7.5ft livingston on craigslist for $200. I can toss it in the back of my pickup and launch it solo. I put a minn kota on the back and I'm gone. Your definately gonna want something bigger for two people, and I would recomend getting a boat with a v hull rather than the dual hull style. It's a bit awkward trying to move around in a double hull boat. Most importantly your gonna need to pick up some paint and slap a name on your boat. I named mine "Whail Tail"
I have a little 8' flatbottom and have had two people in that, both flyfishing, although level of comfort could be questionable. Nevertheless, a 12 footer is more than enough, you'll be just fine. If you go above 12 feet, it gets pretty tricky to get the boat into the bed of the truck, you really need a trailer.
One other thought / option is a canoe. I spent most of my childhood years fishing from one, and enjoy fishing from one even now very much. They're light, easy to load and maneuver, and if you pick 'em right, can be quite stable.
You'll have a lot of fun fishing from a boat, whatever type you wind up with. I'd stick with a shallow V, flatbottom or canoe though, were it me.