So I would have to solder the connectors on a server psu, that could be a problem with my dad's 30y old cheap iron
Yes, and you'd have to know which points to solder on to. Some PSUs are heavily documented and you can usually find the points on RC forums as those people also heavily rely on server PSUs to power their batteries.
One guide you can use is this:
https://www.xsimulator.net/community/threads/how-to-modify-a-server-power-supply-for-12v.5774/You just need to get a PCIE 6pin or 8pin on there and you should be fine.
Depending on the PSU it should look like this when done, with 6pin connectors at the end of the cables:
http://holybitcoin.com/wp-content/uploads/2015/01/HolyBoard04.jpgFinally, don't use high-AWG cables. You'll end up with burned or melted connectors/cables. I typically see 16 or below.
EDIT: As Phil said below you can just use a breakout board with preinstalled cables but soldering can save you a little bit. I prefer the saving money route.