Smaller trailer might be tough for family of five. Although kids might enjoy sleeping in a tent in the great outdoors. That will certainly help.
In today's economy, may be pretty tough to stay within 30-40K for truck and trailer. I don't want to say impossible, because someone will have an example of how they got truck and trailer for spit and a promise.
I would buy the best/newest/most reliable truck I could get. Last thing you want is to break down with family of 5 on side of the road. Make sure whatever you buy has a tow/haul mode and preferably selectable gear setting for the trans so you can have some non-brake control on the downhill side.
I would think a half ton with a 5-6K trailer could work out okay, but as JRScooby says, don't push it up hill, and take it easy coming down.