Plz suggest which GPU should I buy for after effect ?

And after the RAM and disk upgrades, then the GPU. However, your CPU will be the limiting factor as to which GPU that you should get. Among current but outgoing Turing-generation GPUs, I would not recommend going above a GTX 1650 Super or a plain, non-Super GTX 1660. Otherwise, your CPU will bottleneck anything above the plain 1660.

What to do now? Prioritize on both the storage and RAM upgrades before you plunk down a single dollar on the GPU. Any upgraded GPU will be bottlenecked by the deficient RAM and storage, in this particular instance. No single HDD can sustain even 200 MB/s even on the outer tracks, and even the fastest HDDs barely average 150 MB/s. That's over three times slower than even a cheapo SSD.

The media should be on a fast SSD.

Will you have enough room for system files, temp files, cache files, and media files on the SSD? No.

Also, you also will need at least 32 GB RAM for 4K.

Have a look here:

https://www.pugetsystems.com/recommended/Recommended-Systems-for-Adobe-After-Effects-144/Hardware-Recommendations

I would suggest replacing the HDD with a fast SSD and doubling the RAM and then add the graphics card.