

There are server chips like the E7-8891 v3 which lived in a weird middle ground of supporting both ddr3 and ddr4. On paper, it’s about on par with a ryzen 5 5500 and they’re about $20 on US eBay. I’ve been toying with the idea of buying an aftermarket/used server board to see if it holds up the way it appears to on paper. $20 for a CPU (could even slot 2), $80 for a board, $40 for 32gb of ddr3 in quad chanel. ~$160 for a set of core components doesn’t seem that bad in modern times, especially if you can use quad/oct channel to offset the bandwidth difference between ddr3 and ddr4.
I think finding a cooler and a case would be the hardest part


If caddy is acting as a proxy for anything, you should not need to forward that port externally. Local host firewalls allowing traffic on your local network is sufficient.
Depending on your physical host layout you may be looking at an issue with nat reflection.
You have not given us enough about your topology to assist in troubleshooting.