Who-Ha
I have had a 170 for two summers, and I love sailing it.  It is fun, steady and spacious.  However, I usually sail alone or with my daughter.  I've gone out with another adult and another kid (for a totall of two adults and two kids) and the boat was quite full, a bit too full for my taste.  Although it was nice to have the extra weight when the boat started to heel, it is also nice to have some room to move about quickly, especially when there's a gust of wind. When you have 5 in the 170, and you're not saining wing on wing, someone is going to have to sit on the low side.  Not exactly what I like to do.  So, although the 170 is a nice boat, I would suggest that it is a bit too small if you always want to go out with 5 people.As to the motor, 3.5HP is fine, it's what I have, and I can cruise along very nicely if I want.Get the sail cover for use while moored or docked.  Some day you will sell it and the buyer will want it.I know it's more expensive, but I'd look at the 210.  It's received rave reviews from SAIL magazine as one of the top 10 boats of the year.  It would probably be better for a crew of 5.However, I the real crew will be 2 or 3, as I suspect, the 170 is fine.  As for used 170's, the model is only 2+ years old.Good luck.