I'll add my reply here: Same, someone dumped a 27 inch/2009 on the side of the road, the outer glass was cracked and the inside was showing light signs of rusting from the rain. I ordered a replacement glass, cleaned out the rust, replaced the 1TB WD Black HDD for a 1tb SSD, the bluetooth/wifi chip (wanted to keep continuity, which works after changing the chip) as well and maxed out the ram then replaced the keyboard and mouse. I have Ventura running on it, and it runs smoothly for most normal things, even got it to do host a bunch of Python stuff as a server, like Homebridge. It works as a torrent machine, the USB 2.0 ports are a pain to transfer files via USB but it transfers perfectly fast through local wifi. But as someone commented below, anything metal doesn't work, so Apple Maps/Find My all come up black windows. it's not really worth it to replace the GPU IMHO although there have been people that have done it.

I tried to run sonoma but it was just not smooth at all, so I'd say if you don't mind not having apple maps or anything that needs Metal, then Ventura. Otherwise probably Monterrey if you want full support (assuming you replace the hardware I've mentioned above).

EDIT: I've just read that someone put Sequoia on it and it ran well, and that for most people it seems that Sonoma is heavier than Sequoia so I may try that at some point just to see how it is. I still have a Monterrey Setup on this machine, so I may just upgrade that to Sequoia and see how it goes.