Prepare yourself for a series of people extolling the virtues of their own particular SIB. Me first
Is 2.4 to 2.6 metre going to be too limiting? It will get increasingly cosy as the 'pups' grow up.
Is 4HP going to be enough? Fine for the river but if you want to travel a bit further afield / be able to plane...
Name wise you are looking at Avon (hypalon) or Zodiac / Quicksilver / Honwave / Suzumar (PVC) or a wide range of 'no-name' SIBs (PVC), eg the Saturn SIBs discussed here:
You can buy an old Avon SIB for £150 (Avon Redcrest etc - make sure you get an outboard bracket!) to £400 (Avon S250 / Rover etc) from all the usual suspects (Ebay, B&O, etc). They last extremely well.
4HP outboard maybe £200 to £400 depending on age / condition?