Check out the West Marine Rigging
Advisor. I expect you will find that the loads in your situation can be handled by line smaller in diameter than is comfortable in your hands. Don't go too large or the line won't run smoothly in your block and tackle.
I have a fairly similar dinghy
with a 9.9 2 stroke
outboard). I am using the core
of some 3/8" NE ropes sta-set for my bridle as I had it laying around. 1.5 years later it is still working fine.
If you look at most industrial lifting rigging
these days you will find most is no longer done with cable, but rather nylon straps as the later are far more capable of damping shock loads. Think of the bounce your dinghy
might take suspended from the davits riding over a large wake or in rougher weather
before you order a set of cable bridles.