Just to let you know that the solar panels work also if it's cloudy, of course at 50% of the capacity, but still work! We payed our first system (80W solar panel, regulator & inverter) around 250 Euros. Then our upgrade with 2 new 100W ultraflex solar panels and a new inverter for 1Kw was around 600 Euros. https://www.youtube.com/watch?v=VElVti_FtkU&t=1s
About the water tank I think that 250 Lt is more than what you can actually consume. We have a 80 Lt tank in the back and it's ok for a week with cooking, drinking, cleaning and showering.
About the connection for the genny and the batteries I can't say much as we don't use it but here maybe you can find some help: https://www.solarpaneltalk.com/forum/of ... ttery-bank