From my experience the most fun was had with the smaller boats. Now with that said what to look for?
The deeper the boat the safer the boat! I would rather 12" of free-board than 6" less chance to take water over the sides. Decking amounts to more free-board. Engine size, The bigger the engine the more it costs to run. With that said you need to match the power correctly with the size of the boat. I had a sixteen footer with a 25 hp engine and updated to a forty and the fuel consumption went down. Shallow lakes can get large swells so I would recommenced to have a high front to keep out the water. I had the bow raised on my boat to five feet and on the lake I used it I took green waves over the front. The wider the boat the more stable, again if bilge keels are attached then the stability is increased. Drop center boards increase stability. If you go slower you see more and on the water time doesn't count as you have all day to get there. Most people these days have the hurry worry disease and its fatal. No hurry no worry. Now if it takes more than a day to get there then you need a boat that you can sleep on. Then there is the weather rain or just the heat of the sun or the wind so some sort of cover is needed.
With this boat you will also need to get back in the boat if you fall out. Must have a ladder available at all times. Then if the motor breaks down some sort of auxiliary power is needed so a small sail or a kite is more efficient than oars. Water is collected from the roof, if you have one. Its all about self sufficiency as you just don't get out and walk home when things go wrong.
The must haves are emergency radio beacons on the person as well as buoyancy vests as you don't know when you might end up in the drink.
If you are out at night navigation lights are essential and red interior light to keep night vision.
I will add to this post as things come to mind, The boat described above is my drug sub but you might like other features so give it a go. My little 8 foot tug boat could take waves over the deck and hit the wheel house and remain dry inside where a open boat would have sunk. The best boats are not necessarily the spruced up expensive boats.
In Sydney Australia they made a little motor called the Chapman pup and was 2.5hp. I was looking for parts for one of these and met the maker of this motor who took it on a voyage up the east coast of Australia and it took several months to see if it would last and it performed exceptionally and this was in a sixteen foot open boat. I can't think of the blokes name but he was in his seventy's when he told me of the story, I must admit I was Humbled in the presence of this man.