Everything revolves around the money you want to or can spend. Jigsaw had some nice ideas if they fall withing your budget.
Last year or the year before I got my father in-law a Cabelas Depth Master Gold. It's not a real expensive reel but it works and he likes it. For trolling it is nice as he does not have to judge the amount of line let out, as you may know that makes a big difference for lure depth if not using down riggers. There are also other nicer line counting reels on the market.
As another idea Cabelas also sells a couple Precision Trolling guide "The Troller's Bible" books that helps you determine the depth of certain lures according to line out, line diameter, and speed. My father in-law likes his. They also have down rigger fishing technique book and others. Just trying to give you some ideas. Links to the Troller's bible books below.