Need advice: I want to set up my own firewall and SEIM.

I want to set up my own firewall and SIEM. I am relatively young and almost all of my networking knowledge has come from working at a very large enterprise out of college so I could use some help with home networking. I came into work and showed some coworkers a sketch network with 2 firewalls and they laughed at me. Iv been at it for a few days and I really just need some advice.

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Firewall:

My friends tell me to just use an old SFF dell optiplex or something for the firewall. [Pfsense or ipFire]

However, my research has led me down the path of ubiquity products. The Edgerouter X and Lite are very popular. Why would I use those instead, just because I would have to get more NICs in the PC to use it as a router?

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SEIM:

This should be separate and behind the router. I am looking at a used HP DL360 G7 or a Dell R710. 1 or 2u. 48gb x2 X5650 6 CORE 2.66GHz

Is this a good understanding of the hardware requirements I would need to set up this network? And as I understand it, the flow of traffic would be the fallowing:

Internet -> Modem -> FW/Router [Optiplex] -> lan/wifi/SIEM