Matt,
The two best days don't have to be the day you buy and the day you sell. Unless you are selling to get another boat. Most boat ownership issues revolve around unrealistic expectations and buying the wrong boat.
First boats while fun are a lot of work, much more than our cars. Know that going in. A marine engine is under 100% power 100% of the time so they break and need maintenance much more regularly than a car. Be prepared to do regular and proper maintenance. The elements are harsh. Fortunately you won't have salt water issues to deal with so you are a step ahead of the game. Do your self a favor and set the ruels for maintenance and cleaning the boat from day one. On a well run boat everyone has a duty and everything has a place. It will make launching and hauling/tieing up at the end of the day and cleaning much easier. Dad is usually the guy at the end of the day cleaning and putting everything away until 11PM when everyone else is inside so nip that in the bud from the get go. Same goes for guests they have to pitch in. Give everyone a job. With 5 of you clean up will go fast.
Buy the right boat with proper horse power. A 16-20 foot boat for 5 people is going to be to small right from the start. You will be over crowed with no room to move before you even add a cooler, towels, tubes, ski's or guests. In addition the boat you show has an older Evinrude 115 (2-stroke) It is going to need a lot of maintenance from the day you purchase it. You will spend more time fixing the motor than using the boat.
Being on a lake you do not have tides, currents to deal with (as long as you are not on lake Erie or something) so you wont need the extra horse power or manueverability to over come those forces.
For what you have stated you want to do and the body of water you will be on I would recommend a deck boat starting at 24'. Lots of room and open space. I have never been a fan of I/O's as all the working parts and seals spend to much time in the water being attacked. But I boat on salt water not as big an issue for you. But an out board in my opinion is much better when trimmed up everything is out of the water. It's also easier to work on.
I personally have never purchased a new hull. It will take 30 years for a fiberglass hull to go bad and even then it has plenty of life and can easily be repaired. So I would look for a used hull that's a few year old. They are cheap. If it has an engine sell it and buy a new engine. Look at the Coast Gurad tag for the maximum horse power rating and put on as big an engine as the boat will allow. Your fuel consumption will be better than a smaller engine, The wear on the motor will be less and you know you have excess power if needed. The new Evinrudes are awesome. The company is owned by Bombarier (See Doo, Boambardier Jet) You will get new technology a warranty and a large dealer base for parts and repairs. I am not a big fan personally but Mercury is know for being a fresh water engine. Same applies new engine, warranty and you know where you are starting.
That makes for a good time on the water so you can enjoy from day one.
Here are a few deck boats. Do not get a pontoon boat thery are not the same.
