Given the two kids scenario, I think you'll soon be very disappointed in a 3.1m boat. Try looking at a second hand Avon Searider 4m like mine, which is a fantastic starter boat and considerably more seaworthy than the 3.1 Avon Rover RIB. A lot of the experienced guys on this forum started out with a 'baby' Searider and a lot of them will tell you they had more fun with it than the larger RIBS they've subsequently bought.
I've seen about 3 advertised in the last couple of months within your budget, but don't make my mistake - make sure you get one with a 40 or 50 on the tail, because likely as not you'll discover a liking for water skiing and if you end up with a 25 or 30 you'll be very frustrated.
If you need any technical advice appraising a second hand Searider I'm sure Rogue Wave or David Manning on this forum, amongst others, would be very willing to share their extensive experience with you.
ps. forgot to welcome you to the forum, and looking at your user name it would appear we have yet another Land Rover owner in our midst - yippee
If you've got a spare afternoon may I point you in the direction of the mammoth thread on off-roading started by Jono Garton under the 'Other Stuff' forum.