I can't address the combination of a Benford Dory and vane gears from personal experience, but I do have a long history of using vane gears on other vessels.
In general, trim tabs don't steer as well as pendulums, but are adequate on long keeled boats. Pendulums will steer anything but even they will struggle with modern fin keel/ balanced spade rudder underwater profiles, and it takes the extra little twist of inclining the horizontal axis of the pendulum and sweeping the blade aft, as on the Windpilot, to cope with these.
But given a broad-ish keel and a rudder without too much balance, an Aries will cope. Nick Franklin made several different designs of Aries, but only one of them was manufactured in large numbers, and I guess the one available to you is one of those. They were all intended to steer boats of about 30 - 50ft LOA. It's not the only gear with the configuration of inclined-axis vane and vertical pendulum, the Monitor and others will steer as well. I don't particularly like the course setting of the Aries, which works in 6˚ steps rather than being continuously adjustable, but in open sea passage-making conditions, that doesn't matter too much. The only problem with a Benford Dory is fitting it to a stern that doesn't have the width of a transom, and fitting it aft of the outboard rudder. These problems are soluble, with long enough mounting tubes, but it does put the gear rather out of easy reach.
But to address the immediate question: if there is an Aries going for next-to-nothing, seize it with both hands before someone else does! If it's old, the bearings may need sorting out, but otherwise it's worth pursuing.