I am going to inspect a used 8 GPU (Sapphire RX580 8GB) mining rig with a single 1300 Watt EVGA PSU (I'm assuming its the 1300 G2). He claims he gets 29 MH/s per card. It is running on Windows 10.

That sounds a bit underpowered for 8 RX580's

The seller said the RX580's are all undervolted and bios modded, so that it is safe for the 1300W PSU.

What are your opinons on this?

Thanks for the advice. You might just be saving me from a costly mistake if this is not good. lol.

*edit he also claims he was able to get 50% undervolting on the cards (not sure if he's talking about bios mod or in msi afterburner.) He says he uses both.

**Any other basic checking I should do to make sure its a good purchase? (I will verify booting, and hashing for a few minutes to makes sure it hashes correctly.

I just want to make sure when I'm in a mining pool and using CLI CGminer that I'm sending the payout to the right place!

For example:
My electrum wallet has multiple "receiving addresses" they all start with bc1xxxxxxxxxxxxxxxxxxxxxxxx.

Is that what I put in as the username in cgminer?

For example, is this correct?
If my receiving address is bc1XXXXX, then my bat file should read:

cgminer.exe -o stratum+tcp://pool.ckpool.org:3333 --suggest-diff 128 -u bc1XXXXX -p x

username "-u" is basically the entire receiving address in electrum wallet? And I can use any one of them since there are multiple ones?

Thanks for your assistance.