Hi Stuart - I'm one of many who will appreciate many of the points you have made and who went from a RIB to a SIB for the very same reasons.
I would recommend spending a few evenings reading back through the SIB section for loads of topics but consensus is normally that:
3.2m (ish) is the smallest practical size SIB for typical 1+2 or 2+2 coastal use.
A 9.8 to 15 HP 2 stroke is the favoured engine - figure 16 - 18 kts as a typical WOT speed.
You will get wetter than in a RIB.
Being perfectly honest anything bigger than a flat sea is a PITA (literally) in a SIB (seas a RIB would laugh at are not much fun).
But for £1500 to £2000 you get a heck of a lot of fun that goes in the boot of a large car or on the roof (my favoured option), is a cinch to store and maintain, can be packed away in the corner of the garage, costs peanuts to run (£60 insurance), £10 fuel for a few hours at (mostly) WOT, can be launched by one or two people literally anywhere - any beach, lifted into a river or lake, etc, etc.
SIB's are not for everyone but for convenience, low capital outlay and real fun they are hard to beat. And if you run out of fuel like I (stupidly)did you drift onto the beach and carry it up the steps to the main road and pop it back on the roof of the Landy - not something so easy with a bigger boat...
Just search eBay for a week and you will see loads of suitable sub £2k rigs - buy a well known make/combo and try it out. You will be able to sell at no loss if you buy right.
And if you need more convincing check out wilk's video he posted of his 25 nm trip.