Non locking Flex head is the most useful, I guess. It can be flat and offset. Obviously, it is the most versatile wrench. But I don't like Locking flex head. Hand tools have to be simple and sturdy.
look great!
Where can I get trays? Probably, I need at least 15 trays in total (for all drive sockets). Is there the place to buy in bulk with discount?
You may wanna consider Channellock internal external convertible pliers. They are very sturdy. If you don't use pliers in narrow space, they come in handy.