Re: Thinking about pontoon. Am I crazy?
Congratulations! Thinking you're into it right, and now, with just a little work you'll come out smelling like a rose!
Re: the 19" tubes, on inland lakes that don't get real rough, and 4-6 people, I kinda like them? They ride SO much better (with a reasonable load) the difference is very noticeable! Where the bigger pontoons refuse to do anything but sit on the water's surface, the 19's are small enough to actually penetrate waves, smoothing your way? Proper balance is more important though. Pretty easy to get the back end loaded up to the point it floats too low, or too many big guys up front causing the opposite? Keep that in mind if you're considering relocating the motor mount, deck railing (forward/backward location on the deck?), and console?
Worst case- If you do a nice job, try it and it doesn't work out to your satisfaction, you can very likely sell it for more than what you have into it, and use that towards the next one? The trailer is likely worth nearly what you have into the whole thing?
Absolute best of luck! -Al