David - toothed belts can be very noisy as Alan says - it depends on the belt speed and configuration so is designed out of "normal" engines. I think the noisy speeds are called idiosyncratic or similar. Our engine
has a toothed timing belt and a peculiar very low level whine at some speeds which I attribute to the belt (I hope
For the original poster I too think Vee belts are the best for the application described and while it is dependant on engine speed (belt handling kW decreases with lower engine speed), pulley configuration and the loading up characteristics of the pump and alternator
you may find an A section belt is ok for each of them. Under normal conditions a properly designed A section belt should handle 7 or so kW at idle engine speeds (but see below re design, don't take my word for it).
Also, Vee belts are always readily found and one should use a good make such as never use anything other than Gates
There is a common belief among amateur yachtsmen that for 100A plus 12v alternators that 2 belts are required but that is not so. As an example some recent big boats I was involved in had 2 150A 24v alternators (so twice the power of a 12v alternator) on each and they were driven by a single
belt (but maybe a B section not an A, I cannot recall) with no problems. In fact the belt was so small looking in comparison to the alternator that I asked the ship yard mechanical engineers to recheck it - it was fine and as far as I know has proved so in service
But whatever you should get the design of the belts performed by a design engineer
experienced in drives and not by some backyard "mechanic". You should also ensure that your engine's bearings are rated for the overhung load of the added belt drives.
As an aside on belt tightness, there has been a big move to flat ribbed belts in the auto industry - I am surprised at how tight these are set up on new cars even though there seems to be little lifetime stretch in them but I am sure they know what they are doing