Industry standard for M10 cap heads is 8mm hex key, if you're going to replace them, look for 10.9 stamped on the head which indicates a higher tensile strength. BZP (bright zinc plate) finish should be fine, but if you're using S/S nuts and bolts make sure the threads are clean (use brush & brake cleaner) as they are prone to galling (sticking). It's probably worth using a decent fastener supplier such as gwr-fasteners, spalding, namric etc. instead of no-name Chineseum off Ebay.