What is the best boat that $30-35K can buy for our needs?
I don't know much about the Ohio river, but if that's where you are going to sail, you ned to find out about things like current, prevailing winds, narrowest points, etc. and especially, average depth, if you are going to get a boat of any size. What immediately comes to my mind when I read about who is going to sail, is that you would do well to look for weekenders with large cockpits and small cabins rather than boats that are designed primarily for cruising. You spend 90% of your time in the cockpit, and if there are more than four on board, most boats get a bit cramped. You need more space on a sailboat than a powerboat, as weight needs to be shifted and people have to get out of the way of lines and booms, etc.
Boats with nice cabins usually don't have large cockpits until you get up to about forty feet, and those boats are out of your price range. Catalinas are decent boats, but I think you're going to find the cockpit a bit cramped for everyone.
What is the real performance advantage (speed) of a tall rig vs. standard rig?
The tall rig boats are meant for areas where the winds are lighter, the Great Lakes for instance. The standard rig boats are more suited to windy areas such as San Francisco Bay. There is no constant correlation between the amount of sail area and speed. In light winds, iut is likely that the tall rig will do a bit better, in heavvy winds, the standard rig will do better, as the boat will not heel as much, hence you will not need to reef as early.
Why is it that I can find a better deal on a Catalina 30 than a 27 or 28mkII in the same year range?
I would guess that it's because there were so many of the 30's built. It is one of the biggest selling sailboats of all time.