What to use X100P, SPA series or?

New to asterisk/ telecom in general
Confused on what hardware to purchase

I’m looking to install an asterisk pbx and not sure if I need a “modem” card a la x100p or would a voip type router such as a linksys spa series be the better choice?

The details are 1 analog phone line (pstn) coming into the office (with possible expansion to 2), 6 workstations that need to be able to call one another via pbx/voip and also send/receieve calls from the pstn line.

I’m planning to use a linux server for the pbs that will handle the call work load call volumn very low, perhaps 50 per hour.

any thoughts