I’ve just replaced my HDD with SSD and have done some research on this, so here is my 2 cent.
First thing to consider is what kind of form factor you are looking for. SATA 2.5" SSD is the most common type, you can stick in a desktop and oldish laptop with zero problem. New systems now come with the M.2 form factor, which is just a stick, like ram. It is especially popular for ultrabook since you need a lot less space. M.2 form factor comes in two speed. First is SATA, which is slower, or through PCIe, which is faster. Through PCIe you can get an even faster speed with NVMe. So check which form factor and interface your mobo can support. Price wise M.2 SSD and 2.5" SSD cost the same, but you have to pay a premium for NVMe.
For a boot only SSD, you can get away with as little as 250 GB. But having a hybrid SSD+HDD config is for the age where SSD is unaffordable, so you use HDD for storage and SSD for boot. Now with SSD reasonably affordable you may consider just one big SSD with 1TB or 2TB, especially if you are coming from having 2 2TB HDD. Price of a 2TB SSD however is quite steep.
The compromise I find useful is to buy a 1TB for boot and games. If you run out of space (a big if) you just buy an el-cheapo 1TB or 2TB SSD in future for storage (and in the future the price may come down even more). The boot SSD you definitely want a quality SSD because reinstalling windows is PITA. With cloud storage there is just no reason to download lots of games anyway. Now remember new game is now usually ~60GB, so 1 TB easily fills up, going forward. Another compromise is to keep one of your old 2TB HDD for storage, and copy a few games you play at the moment into the boot 500GB SSD for faster speed.
As to which brand to choose, Samsung Evo 860 is by all consenus the most reliable and fastest for desktop and laptop use. Evo 970 is even faster, but uses the newer NVMe interface, and it isn’t cheap. Crucial MX500 is the cheapest, but by all consenus it is good enough for most intents and purposes, even for a gaming rig. 2TB Crucial MX500 SSD still isn’t cheap, especially if you compare it to a 2TB HDD, which is dirt cheap.
As to migration, moving from a smaller drive to a bigger drive is easier than from big to small. From small to big, you can just clone the drive first (Samsung and Crucial come with cloning software, not sure about other brands), then use windows disk manager to expand the space. From big to small you have to first uninstall stuff to fit the smaller drive, then clone. Moving between drive of the same size is the most painless, you just clone and go.