That sounds more like a SIB (fully inflatable) if its new for that Budget. As Nos says even 2nd hand you are heading towards the old/needs work category for reputable brand of rib. If the kids were older then sitting on the tubes might be an option.
Rather than tow all the way to scotland you could keep it there and then towing 900 miles per trip would be less of a concern? (that would probably save you around 45 mins on the trip, and fuel)
My experience with children that age is:
- their attention span is short. So the boat is exciting - but only for the first 30 minutes of any trip.
- they get cold much quicker than we do
- they don't like the wind in their face (a boat doing 20 knots is a lot of wind in face)
- they object to salt water spray in their face.
- they won't like choppy seas in a small rib
Actually many of those points apply to my wife too!
The problem you then face is that when a nice day comes along the boat is too far away for that trip. When you plan trips half the time you can't go / cut short the plans etc because of the weather. So actually for the sort of boating you are describing do you really want a rib - or would something with a cuddy be better (won't look as good or go as fast, but would probably be more practical).
Sorry for being a bit negative. However I was in a similar boat (excuse the pun) to you 3 years ago. This is what I ended up with... it is a plastic boat (made same way as many sailing dinghies) in the style of a rib (but not actually inflatable). There is ample seating for 2 adults and 2 kids. The engine (only 20HP) is powerful enough, will plane no problem and would probably even pull a ringo. Tows well. Not easily damaged so good for dragging up a beach (a bit heavy without wheels), bumping a few rocks on a West of Scotland beach or just pontoon bashing.
Could be done for your budget new, or quite a bit less if you wanted to buy mine! Officially mine is not for sale - but because of the issues I highlight with kids (mine are 4 and <1) its not getting used enough. With other commitments in life I can't justify the cost of a bigger boat all to myself which would provide the shelter required for West coast cruising with a young family.