One thing to keep in mind if you go with an i5 or i7 is that you won't have the option to use ECC memory. If you're running TrueNAS, you'll need ECC memory for the ZFS cache. A Xeon E5 v2 server is old, but still has a more than enough power for your use case, and they're not particularly expensive.
If you need something more powerful, you can find some decent Xeon Gold systems on eBay, but they'll be a bit more pricey. The new Xeon W chips are also an option, but at least for me, they're prohibitively expensive.