Re: first time buyer,
Don't pay much for it!
If you find yourself having to swap engines, replace decking, adding canvas tops, and adding furniture, you'll be spending $5000-$6000 on top of what the pontoon costs. For $6000 you can get a water ready pontoon and just go boating.
Maybe you just buy it and enjoy the current setup. That motor better start flawlessly, shift, and be powerful across the RPM range. If it looks all hacked together, I would stay away from it.
Just a couple of thoughts.... I have a 29' pontoon and love the space. So 28' is a good thing !
The 2.3 motor, provided it is in top shape, is ~120HP. That is a nice HP to do watersports if that is part of your plan.
If there is a hardtop, big bonus. You'll want some shade.
28' will probably have 22"-23" pontoons, so that is good too.
It will have built in gas tanks somewhere. See how easy they are to access. Being a 1970s boat, things like fuel lines, tank crud, broken senders etc. come to mind. You may have to deal with that and then add a good filtration system if there is not one.
If you have fishing plans and are thinking a trolling motor, you'll need a 24v 75lbs thrust setup to move that 28' pontoon.
Ok, updates us with the results !