What problems will I have when making the change?
1| Front panel connectivity but you can avert that with this;
https://www.amazon.com/Front-Panel-Header-Adapter-Optiplex/dp/B0BWMZSPBY?th=1
2| The form factor to the motherboard is proprietary, even though the connectors on the board are standard, the board's size isn't.
What graphics card do you recommend?
In 2024 for that platform, nothing. Save up the money and build an entry level system with concurrentparts and with the avenue to upgrade later down the road as opposed to dropping money on a decade old platform with no upgrade potential. Even if it was upgraded with yesteryears hardware, you're not gaining much for the moeny spent.
It doesn't even have Windows 11 support while Windows 10 will be a decade old in 1 years time.