If you have the email configured, the receiving and detection of the fax is pretty easy and is done in “inbound route” settings in like 1 click. T.38 is supported by Asterisk, but not all (if any) SiP providers do support it. In probably 90% cases, Fax can be detected and received by the FreePBX Asterisk without T.38 support, but it is available as a check mark in the FreePBX settings. Archiving however, is not easy to do without Fax-Pro module.
If you plan sending faxes from a Fax machine with ATA adapter, you can and I had that in our office for a while, but now we use FAX-Pro module which is way better way as it allows send faxes via User Control Panel right from their computer. We have actually sfax machine at this point.