I have an 18' Tracker Targa and I just bit the bullet and bought a new 9.9 Merc Bigfoot and I do not regret it at all. It is the best purchase I have made yet. I could not find used ones on Ebay or Craigslist anywhere near the Toledo area, so I went to BPS and ordered the motor I wanted, paid the money, and I will tell you it was well worth it. 25" shaft is a good idea, it fits an 18' aluminum perfectly when mounted to the transom and not a lift. Also, Trollmaster makes a good digital throttle control for about 200 bucks that makes speed control a breeze, I would highly recommend it. For steering, EZ Steer makes a bracket that connects your main motor to the kicker that works well. For now I use that, until of course I get the money for a MinnKota PD 70 with Ipilot Just thought I would share my suggestions, it isn't cheap but it works like a charm.
Dan