just my 2 cents but you are better off buying new rather then second hand and end up with someone elses troubles or have it crap out on you in less then a year.
one important thing to consider is how many batteries you want for it since most are for use with 2 or 3 batteries, and only a small few are for just one battery so 50-55 lbs is about the biggest you find that's for 12v. if you want to keep weight down and only use one battery you only have one or maybe two to choose from.
if you have room for two trolling motor batteries then I would look for around 70-100 lbs. of thrust so it can handle a little wind or strong saltwater tides and a light chop fishing the bays.
I have the Motorguide X3 on my Aweld 17x52 with Suzuki DF40 and it works great but on windy days (10+ mph) it struggles a little unless im on high speed so I have to go sideways or with the wind to run on #2 or #3 setting. it pushes the hell out of the boat as long as I don't have to fight the wind or try to move against a fast tide.
seen here:
http://motorguide.factoryoutletstore...egory_id=24611