Nobody is using MMF these days even for local runs. As to sfp, check https://fs.com and pick a matching pair that is cheapest. These days it makes sense to use 10G or 25G rather than 1G. Some people run 100G for their homelab, but even used it’s pricy and noisy.

An L3 switch is a router. Though most of them don’t have enough resources to take a full BGP routing table, at wire speed.

Why not postfix?

I bought my StarTech full size frame rack on Amazon, since it was cheap and disassemblable. Ditto for network wall rack in the basement, made in Poland. There is a type of music rack on casters which is also good for short equipment, also Amazon. Apart from that it’s eBay and local used equipment sites. Mikrotik is cheap enough to buy new.

What are your specific needs? Mine are low-power and low-noise or passively cooled for 24/7/365 stuff. Everything else is free game, since it never runs long enough to matter much.