Perhaps also consider what your needs are. How many users, how many concurrent calls ?
Maybe SOHO or small business ? Or a large call centre ?
Depending upon your needs, FreePBX can handle reasonable volume of calls with ordinary craptastic hardwares.
IMHO, for small installation, i5 is likely serious overkill.
So grab yerself an old mobo with a few PCI slots, some ram, plug in your analog, POTS, E1/T1, ISDN, whatever card/s, install the FreePBX distro and enjoy.
Stir and sweeten to taste.
When you’re whelmed by how great it is, don’t forget to buy some commercial module/s and support the development team.
YMMV