CrappiePappy explains the differences pretty well.
I never use waypoints as an exact spot, GPS error can be as much as 10' in any direction.
As for used units. It's like anything else, run it through its paces. Pretty sure there is an hour meter on the main display, it will tell you how many hours the unit has been on and in use.
If your sticking to 1100, and you want the waypoints sharing (which I recommend), verify the units your looking at can share waypoints.
I believe all the lakes you mentioned are mapped and readily available, might even be included in the Humminbird base maps they include on the sonar units. Verify that by looking at the list on HB website.
As for your budget. I would seriously consider buying a more expensive unit (Helix), and a really cheap unit (PMax). Plan to upgrade the cheap unit to a more expensive unit next year, or when the budget allows. Make sure the expensive unit meets your needs when you have both units. One of the best strategies I use is to put the word out with friends and family. Tell them you want to buy sonar units and too keep there ears peeled. I get great deals this way, however it takes time, 3-4months if you keep asking around.
HOI Crappie Club
Where family and friends come to compete for a little more than bragging rights.
Quick, someone teach me how to fish so I can win this tournament!!!