If you buy used skimmer, I would recommend one for which you can easily find manual online as they are tricky to tune.
Run it through solution of vinegar or citric acid for a day or so.
One of my LFS has section of used gear, including skimmers, I am sure you could find something like that in Jacksonville.
I can't answer your second question as this is individual choice.DC controllable skimmers are regarded by many as superior, but they are more complicated and benefits may not be worth extra money. I used DC controlled skimmers and "regular" skimmers and they are both fine.