Bensheim,

Are the two machines currently running Tiger PowerPCs?

If so, then they do not support Snow Leopard which can only be installed on a Mac with an Intel processor.
Quote:
. . . then that will just leave upgrading the 2 Tiger macs which are not going to be replaced. Will the instal/system disks which come with the NEW machines, instal SL on them . . . ?

The discs which ship with a new Mac are machine specific. It is unlikely that they will boot and install an operating system and bundled apps on other machines. There is also the Software License Agreement.

I have no experience with network file sharing between Tiger and Snow Leopard and cannot answer that question.

Back up everything you can't afford to lose: documents, mail, movies, music, photos, and other data and settings.