5 users, some smartphones... that shouldn't be a problem for the j1900. It seems to be the best compromise regarding to price, power consumption and speed. The J1900 has a TDP of 10 W, and it's a SoC...
And if the Realtek doesn't work right or performance is too bad, this one from Intel should be ok:
Intel PRO/1000 MT Server Adapter, 2x 1000Base-T, PCI-X 133MHz, low profile (PWLA8492MT) with Intel 82546EB chipset
Alternative would be a 1155 or 1150 board with something like a Intel Pentium G2030 or Intel Pentium G3220, I don't think a core i3 is for the requied amount of user necessary.