Using 20mm means you’re mostly looking at plastic kits, unless Del Prado or someone similar make a built up range. Such things exist for WWII but I don’t recall seeing a WWI equivalent.
If you can cope with the different scale then the WOW/WOG 1/144 ‘planes are a quick & relatively cheap fix. At around $20 for a scout, they’re around double the cost of an unbuilt & unpainted 20mm kit. Milisms carry them if no-one local has them BTW http://www.milsims.com.au/catalog/3309
I’ll add more here later on the other 2 questions – I have to go shopping.