Mac desktop 21.5

I recommend you get as much memory as you can to future proof your machine. Base is 8 GB but 16 GB is better. A larger SSD isn't as important IMO because you can just get a good external SSD if you need more storage.

+1 on more RAM, minimum 16GB, because RAM cannot be added later. What you buy now will be your "forever RAM."

As for the SSD, I would go with a 512GB min. I am seeing too many reports here lately of people with 256GB SSDs having trouble upgrading system software because they did not have enough free drive space. An external drive won't help there except for off-loading data, not apps or system files. Remember, some apps won't work from an external and if they do, the typical USB3 external drive will be too slow for apps. You have to move up to expensive Thunderbolt 3 external SSD's to match the speed of the internal SSDs.

Make sure you get one with an SSD; earlier 21.5" models had a default 5400 rpm spinning drive which was as slow as molasses. It didn't matter how much RAM you installed, it was still slow. As for the RAM, 8 GB is about the minimum with Big Sur, so 16 GB would be a minimum. If you intend to do a lot of image/video editing/rendering, then 32 GB would be better. And, as for the SSD, the drive size should be at least 512 GB (that, by the way, has nothing to do with memory) and remember that you should always maintain an absolute minimum of 20 - 25 GB free/empty space on that SSD for the OS to perform satisfactorily.