Hi Phil, welcome to Ribnet!
Hmmm, Sadler 29? Lovely yacht! You will be horrified to find that a quality RIB 8m+ will blow your budget - do you want to buy new?
Taking Charles out for lunch is a great idea, but he has expensive taste and will probably do his "throwing your wallet in the harbour" routine
There are four reasons for bolting twin engines onto the transom of a RIB:
1. You intend to cruise, extensively, out of 'phone and VHF radio range or in dangerous water, on your own (ie no accompanying boat).
2. You can't get the horesepower from a single engine.
3. Your budget's unlimited.
4. You have free fuel, or live in the Channel Islands
8m+ RIBs are great - every half metre gives you more comfort, and make those long passages more bearable. Your wife and young children will enjoy it rather than enduring it. And your one-year-old will only just be old enough by the summer to endure the impact of RIBing. You might find that you want to keep to yachting for a couple of years!
With a budget of £30k you've only one option - used. A new 8m+ with twin engines will set you back £60k+ from a quality manufacturer.
As Roy mentions, Ribcraft are a quality manufacturer with a good reputation. We happen to be very satisfied with our Scorpion, and I can honestly say that along with Tornado (and XS), and Osprey, these four makes set a benchmark that other manufacturers try to follow (unless you can stretch to a Pascoe!).