I'll echo what H20Wolf said and add my knowledge/experience.
I currently own a 16' Tracker Grizzly and a 40/30 Johnson tiller jet. The reason I have this current set-up is because I can take the jet off and put my 20hp tiller prop motor on the Grizzly for the limited hp lakes. If I did not have that need and only fished the river, along with unlimited hp lakes, I'd have something at least 17' and minimum of a 90/60 jet.
My current river boat is my third. My first was a Lowe 1652VPT with a 50/35 jet. Second was a Lowe 1862VPT with 115/80 jet and I loved that boat and to this day regret selling it. I also currently own a glass bass boat. My current boats revolve around me fishing bass tournaments and my needs for those, along with some recreational fishing.
I know the Tidioute area well and have basically fished from the Kinzua dam all the way to East Brady so I've seen just about every inch of the shallow portion of the Allegheny.
I've also been on Tionesta and Kinzua lakes. If I were in your shoes, I'd be looking for something in the 17-18' range with jet drive, and a minimum of 60 hp AT THE JET, and of course that depends on boat size. Please take my advice and go as big as you can possibly afford, both with the boat and motor. You'll always be looking for more space and power.
A popular trend right now are the inboard jets, seeing many more every year, and those run about dirt shallow. The downside to the inboard jet boats is price, they are usually significantly more that outboard boats. Having more power with a jet is significant for a couple reasons, you'll get on plane much quicker (especially important in shallow water), and be able to stay on plane even if you back off the throttle a little (especially important because jets are gas hogs!). Bigger and especially wider boats draft less so you'll be able to drift through shallower water when fishing, plus they draft shallower when on plane, allowing you to run through shallower areas and believe me, there is some really skinny water in the Tidioute area.
Trolling motor........go at least 24 volt and 70 lb thrust like Wolf said. Mine is 24V and 74 lb. It will allow you to hold in the river current if necessary and also have enough power to navigate you quickly when floating with the current. That's important because as you drift you'll encounter shallow areas and rocks that you'll try to avoid. I hate the "power drive" style TM's and prefer cable style. The PD motors rotate way too slow. With a cable style you'll get to learn by foot feel what postion the head is facing and can make immediate corrections without even looking at the head of the motor.
Like already mentioned, Wiegels Brothers in Franklin is a good place to deal with, they deal almost exclusively in jet boats and they know their stuff. Also, the Harrisburg area Craigslist often has jets. Watch in the Tidoute area, there are often boats for sale along the roads up that way. If you do get a jet, or prior to, befriend someone who has experience with a jet on the river. They can cut the learning curve on how to navigate the river and what to look for when navigating. You'll have to learn how to "read the water" which is all important if you want to keep the boat and motor in one piece.
Feel free to ask if I have raised any questions and I get to Tidoiute quite a bit to fish in the summer months. My Tracker Grizzly is camo and the outboard is Johnson gray. Feel free to stop me if you see me!