The internal disk size all depends on how much you are willing to spend and if you are willing to add external storage for your data. Personally, my belief is to maintain a "lean" internal drive with just the OS and applications and then maintain all my data on an external drive or drives. Occasionally if I need some particular speed boost with some disk intensive work, I will keep work in progress files on the internal and when the work is done put it on the external. But if you abhor external drives, get as much storage as you can afford to buy on the internal.
FWIW, with my methodology, I am only using around 120 GB of my internal storage and around 2 TB on external USB 3.2 or Thunderbolt 3/4 drives.
As far as RAM, it really all depends on how you use your machine and the sort of apps you use. If you do basic browsing, email, word processing, etc., 8 GB will be more than enough and on the rare occasion that you may end up with RAM swapping, the internal SSD is so fast you will not notice any slowdown. However, if use apps that are memory hungry like Adobe Lightroom, Photoshop as an example or web browse with dozens of tabs open or run several apps at the same time, again you will want to buy as much RAM as you can afford.
One last note on SSD size, at a minimum purchase a Mini with 512 GB. the 256 GB version only uses a single chip and therefore will be a bit slower than the larger sizes which use two chips. Then again, depending on your use and the types of drives in the machine you are coming from, the 256 GB may be more than fast enough.
