This link will give you an idea of what's available and the relevant prices:
https://www.astounded.com/buy/pa-audio/complete-pa-systems/outdoor-pa-systems/
From that lot, I'd ignore those rated at 30 watts, as they haven't got the power to cover the distance you're talking about. Much the same goes for the 60 watt models. I'd regard 120 watts as the minimum power rating for your needs, so something like this might be suitable:
https://www.astounded.com/shop/field-sound-3-120w-weatherproof-outdoor-4-horn-pa-system/
While I'm certainly no expert in the field, and I know nothing absolutely about that particular firm, that looks like a bargain price to me. (We were paying that sort of price, or higher, for equipment for our school over 30 years ago). You'd need to add on a bit (perhaps £30 or so?) for a second microphone but it looks like a good deal to me.