Hi Chris, their have been many posts on here about aux engines, do a search.
The idea of 1HP per meter is quite solid but I am carrying only a 6HP 4 stroke on my 7.8. The reason being as boats get bigger the HP needed makes them to heavy in my opion to fit to the transon when needed. This is even harder with a diesel as you cant just lowerit onto the transom but need to cross then engine box and lower it onto a bracket.
I mave made a pulley system up to the A frame to make the job easier.
As far as speed goes, as long as you have fuel you will get their. I used to cruise yachts and 3.5Kts was somtimes our max speed in light winds, you just are more aware of currents tides etc and use them to your advantage. When last did a RIB owner plan a passage based on tidal flow atlas
. If you have 5Kts of tide against just anchor up and wait, you will soon have 5 Kts with you.
As far a 4 stroke V 2 stroke
The main differences is weight, although out 6HP 4 stoke is not much heavier than a 2 stroke at 27KG. The main down side of 4 stroke is the oil, not 2 stroke oil but engine oil. Often they can only be stored one way up without the engine sump oil draining out.
As far a common fuel, just carry neat petrol and a bottle of 2 stroke oil. Make a guess on how much you need and add a bit more. If you put too much in and get a smoky exahust who cares if it keeps you of the rocks.
As far as ecomony goes again who cares how often do you plan to use it
and how much fuel do you carry for the main engine.
We have motored from Conway to Fleetwood on a old 2 stroke 8HP in a yacht on 8 gallon.
As far as storeage goes I am not a fan of keeping it on the transom. Many months of salt water spray and damp but never getting run much does not go for easy starting when its needed.
If you have space I think a waterproof bag on deck is better.