What do i need?

Hi all,
New to Asterisk, got a few questions.
Was thinking of setting up an asterisk system in a small office to replace an existing analogue system.
There are 2 phone lines coming into the building and 4 offices connected.
Apart from the server itself, what hardware do I need?
Would I be right in thinking I can use a Digium TDM400P with 2 FXS ports for the external connection and 4 Linksys SPA VoIP phones on the inside?
Do the VoIP phones need to be multi-line phones or will the SPA921’s be ok?
I just need to be able to transfer calls between handsets, call internally fro one handset to another and provide voicemail services for each of the 4 offices.
Any help would be appreciated.

If you choose to use TDM400P card, you’ll need 2 FXO modules for external lines. You seem to suggest that each “office” uses exactly one phone. Should not need any multi-line phones.