FIrst thing, add up the specs on the volume each thing uses, add up any/all that you want to be able to run at once. Then, compare that max value to the max flow capabilities of the 1/2" and the 3/4" valves. If the 1/2" valve can handle the flow volume (probably not), go with it as it's cheaper. You'll probably end up with the 3/4" valve which can , depending on the design, often more than twice the flow of the same company's 1/2" valve. You shouldn't make assumptions, you need to read the specsheets as there is quite a variation between both brands and lines within a brand.
Personally, I like to be able to set the temperature and never adjust it. This is more reliable with a thermostatically controlled valve summer/winter, full tank/near empty than a pressure balanced valve which doesn't take into account variations in cold or hot water supply temperatures - that's all up to you.
As to how best to select a diverter and plumb it, can't help you there.