1) Why do I see rust on stainless parts of a boat? T-top, rail around windshield and rub rail.
Different metals cause rusting, The grade of fasteners used is probably different than the item they're holding. Even if it's a stainless screw holding a stainless whatever, there are different grades of stainless. Rust happens. Elbow grease and a little time scrubbing makes the rust go away.
2) I see some 20 footers set with 140 hp and some with 150 hp, then of course some with 200hp. What, if any efffect would the 140 have over a 150?
Marginally better fuel efficiency. Suzuki makes a very good 140 four stroke, very dependable. Weight is just about the same.
3) If a boat is dry stored (as stated) why would it have a color change at water line? Bad info there I am guessing.
Bad maintenance habits, and certain waterways have "stuff" that yellows paste wax. Not a deal breaker; Barkeeper's Friend will take the scum off, and remove the paste wax. A good synthetic wax won't do that. Plus, if the boat is presently dry stored, that does not mean it was always this way...
4) Swivel seats or Leaning post? I like both, wife likes seats. Is one better than the other or just personal preference?
Swivel seats can be good comfort, but are usually very low. I like to ride higher. When I had swivel seats on my 19'er, I put taller mounting posts on them and that was a big improvement. One time I was out in 4 foot seas tied off to an oil rig, and the only way to stay in the boat was to sit. Took the cushions off, and used the hole in the seats to act as a third hand to hold the rod butt.

Fighting chairs on a 19!!!
to be clear these things are seen on 19 to 21 foot CC boats with single power. I have come to a conclusion that a T- top is a good thing. Always wanted a bimini before.
T tops are vastly superior: for spray curtains, mounting accessories, grabbing on to when pitching and rolling. Only drawback is overhead clearance and storage space limitations - they don't work with an 8 foot door. Just make certain that the T-top is properly built with numerous cross-braces and properly mounted with bolts and backing plates - not just screwed into the fiberglass
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