Which graphics card should I go for playing Gta Sanandreas Multiplayer (SAMP)?

Hard to say without knowing the rest of the system.
If you really dont expect to play other games,
then buy what is needed to meet the recommended specs,
which is a GeForce 3 or better.
The challenge is, even if you find one used, you wont be able to run a modern version of the display driver.
You will need to run the last driver that supported that card. This introduces serious security concerns because malware can and dies target vulnerability in legacy drivers.
Because of that, I'd recommend a gtx-950 as a minimum.
It is still a supported card and shouldn't go out of support too soon.
But if you get a gtx-1050 you will get a longer supported lifecycle.
And you will be equipped for other games. But that assumes your CPU is modern.
For more information on the game and download links, visit here
https://teknolgy.com/gta-san-andreas-indir/