As far as longevity goes, I have been using the same 8MB official MemoryCard from a launch PS2 my dad bought, with no issues ever. If you have a big collection of games you might run out of space eventually, but deleting save files is pretty easy in the system OS.
I bought a separate 8MB official MemoryCard to install FreeMCBoot on a few years back, but I haven’t used it to save any files. It should work the same, though, despite having the Homebrew on it.
If you live in Europe you can buy all sorts of MemoryCards at CeX, asking in the counter as they usually don’t display them. I guess anywhere else where they sell and buy second hand games they have MemoryCards too.
I haven’t tried any third party ones like MadCatz or the new MicroSD ones, maybe someone else can share their experience :)
Good to know that then! I’ll look for a MemCard PRO as others suggested when I eventually expand my setup to unify both of my MemoryCards.