which server model no an dswitch is req

IN my organisation total 150 pcs
Plant A 80
Plant B 50
Head office 20

The distance bet'n plant A plant B is 20 Km. and head office 100Km.
We are using P2P network between plant A& B.
And head office connect through Internet .
We already connected through VPN .
My issue is that now i want to implement domain server in plant A so plz Guide me which
IBM server i wll purchase with model no?
Which layer 3 switch is required ?
and should i purchase two server ?
FYI [For Your Information ]
we have one ERP Software [sql based]
4 Web best application software.
Antivirus corporate edition.