HI Peter,
There are two components to Datalink: the Datalink Manager and the Datalink Agent.
The Datalink Manager is something that Apptio sets up for you and the Datalink Agent is what sits in the customer's internal network.
The Datalink Agent can be installed on either Linux or Windows (32-bit & 64-bit versions are available). We have gone with a Virtual Machine with 2vCPU and 60 GB of storage and don't have any complaints.
If its possible - I would suggest that you have a dedicated Virtual Machine rather than a Shared Server just to avoid any potential problems like problems with other applications causing automated datalink jobs to fail or port conflicts and so on.
For the server size take a look at the Datalink Agent installation guide.
Regards,
Sanjay V