Do you know if you have plans to cruise rivers for the foreseeable future if you could work out a way of getting it to or keeping on the water pretty well any GRP/alloy dinghy type thing with a 4hp would be a better option.
Compared to GRP/alloy a SIB has poor internal dimension for its overall size and they can be very skiddy compared with a conventional boat when it's windy. At sea with plenty of room a burst of power can be used to offset this but less appropriate on the river.
Finally in my experience around slipways, jetties and other places you might stop on the river there are often more tube damaging sharp bits that at sea.
However if you have to use a SIB carried in the car then something with an air floor (for assy ease) around 3.2m with a 4hp would be fine. You will get this in budget particularly as the season winds down starting now as it's Sept.
Remember most rivers require a licence and many charge far more for an OB over 4hp. It can be as different at 4hp £80/yr... 6hp £250/yr.
