Hello @TheBalconGroupLtd ,
It really depends on the load of the servers and what backend applications will also be running. In general though I would expect the 7401P to be suitable, or a 7351P might be sufficient. Again: really depends on the load of the server.
As far as motherboard vendor, we have multiple partners to choose from and you would be best to determine what additional components you need to find a suitable bare bones platform. Those vendors will have approved memory they have tested, along with drives. I'm not sure what you will need the GPU for as most have a VGA port already (unless this is going to be a workstation at the same time).
Hope this helps.