Correct decision to go for 20kW. Solar panels are on US$200 per kwH wholesale so the panels themselves should only cost about $4-5k. They will last basically forever.
The inverters cost less per kWh as you increase the size of the array so you get economies of scale there.
You haven't mentioned how much battery storage but cheap but good LiFePO4 backs 48v 15kwh packs can be had from China for less than $2000. Get 2 or 3 of them.
I would recommend using 3 x Victron Multiplus II 5kW 48v inverters in 3 phase configuration. They are around $2000 each.
Still haven't calculated everything so waiting for the initial quote to judge where I am at. Not sure who is giving me the quote either and will go and have a coffee later this week with my German friend. Whatever happens the materials will be trade prices.