If you don’t want to open the mini case, you could also use a SSD in a USB3 (make sure it supports UASP) or Thunderbolt housing and keep it external.
In general: MLC SSD’s are usually considered a bit more robust, but TLC is the mainstream today. Read and write speeds should meet or exceed 500MB/s. Stay clear from suspiciously inexpensive offers and stick to renown brands (such as e.g. Samsung as top dog or SanDisk or Crucial).
You can choose last-gen products from those when on sale - SATA-based SSD’s have already reached the speed limit, so last gen is not too much of a compromise (sometimes even the better choice, for having better-quality components from before cost-cutting was applied).
Make it a habit to do backups on a regular basis! If an SSD fails, it’s often much more difficult to recover data than with traditional spinners.