The prerequisites for Post master Enterprise are fairly straightforward.
Firstly, a client install of Microsoft Dynamics GP is required; this can be used by users, but you'll need a launch file which doesn't include the products on the incompatible list and if there are any .NET add-ins from the list, a separate install, rather than a separate launch file, will be required.
Secondly, a dedicated Microsoft Dynamics GP user account with permissions to post the batches required. In their documentation, Envisage recommend using an account with the POWERUSER* role, but I would not recommend using an account with those permissions; a role containing only the batch posting for the types you will be posted, is a far more secure option.
Thirdly, a Domain account to use to run the Windows service.