What an open ended question that one is....
and back to you --->
How long is a piece of string ??
This machine,
How many axes, (4 or 5) ?
their addresses (A, B, C) ?
are the rotary axis positioning (3+1/2) ?
or feed ( full 4 or 5 axis ) ?
table-table etc
If not a 5 axis machine, then you may have a generic post supplied with your Mastercam installation, it may need minor mods to give you the code in a way that you would like to see it, and to suit your shop's standards.
Next step is to ring your reseller, and ask for guidance. Ask him for a basic horizontal machine with a suggested post attached ( he may have access to posts that are not on the disk). As he is there to help, you may as well get him to do the easy bit.
The next step depends on what mods are required, and if you could do them yourself or not.
Now, if it is a 5 axis machine, then you may have to purchase a dedicated post. These can be quite complex, AND the $$$ add up real quick, this should be arranged through your reseller, or you can go to a custom post writting company
( HINT---you should have it included in the agreement with the machine purchase that a dedicated 5 axis post for XXX software is also to be supllied with the machine or it don't get bought ).
This is a cost that is going to happen after you get the machine, why not use it as a barganing chip to get it with the machine. It may end up a bit cheaper, and it will be their responsibility to get the post correct.