to be honest mate i dont think a brand new motor is all that important. your big brands like mercury/mariner, yamaha in the 15hp to 25hp range will go for years and years with no dramas as long as you give it a good service once a year. they are tought little motors and wont give you dramas. something about 3.8m is ideal for what you want. i picked up a 2005 3.85 quintrex explorer used once, 2003 15hp merc and 2005 model oceanic trailer for $4150 with the added bonus of extruded gunnels.
start buying the trading post every week anything that sounds ok mark it and ring up. it could be the first or could take up to 3months to find the right 2nd hand tinny but itll be worth it.
good luck mate