Please advice mac server & os x server for open directory ?

Technically, any Mac model capable of running Mavericks (10.9.x) can be used as a Mac OS X Server system running Open Directory. That's the least of your problems, but there's not enough data in your post to tell if that's enough.

There's a world of difference between running a home server with half a dozen accounts and a 10,000 user enterprise with network accounts, network home directories, etc., etc.

You may or may not need to consider availability (e.g. run multiple Open Directory servers to manage load/failure, etc.) - a small home network might not care, and enterprise would.