You might struggle with a 7hp and might need a 9.8 to plane more comfortably with two adults and a child irrespective of flat bottom or slightly less flat air keel - all depends on weight of course - are the adults 20 st together or 30 st etc?
Check the sticky above for info but look at around £400 - £600 going rate for a nice used big brand 3.2/3.4 SIB secondhand on eBay.
Best advice is to sell what you have and buy the below:
Honwave 3.80IE SIB package
If that is still available it's one of the best deals I've ever seen on Ribnet. Engine is worth £1100/£1200, trailer £250/£300, wheels £100, boat £600 etc.