300L should do. you probably will get change from €500 (not a bad guess david!). A stainless steel one will cost you at least twice that.
I see no point in the extra expense and it really depends how how you system is designed. For example you can have a pressurised system with both copper and stainless, however there are differances.
With copper you can not pressurised before cylinder, but you can have a pump after cylinder pulling water out of it and not a pump/or mains pressure pumping into it.
stainless, you can hook up to mains pressure or have water bring pumped through cylinder. Unless you are going for a mains system (no tank in attic) I would go with copper.
with stainless there is the extra cost of expansion vessel/valves/TPRV.
All depends on your planned/exisiting system.