Well OX, that depends on how you look at it. 
Most (Japanese) games that support FM will still play music when there's no FM found. You'll get the PSG versions, then. Ofcourse, it sounds not as good, but at least you have a soundtrack.
With Konami games, it's a different story; they were originally distributed with the SCC built into the game cartridge itself, so there are no "PSG-only" versions of the music included. Meaning, that if you play a cracked version of such a game without having SCC available, you'll mostly only hear the drums and some basslines or echos here and there. 
So, if you plan on generally playing cracked games anyway, I suggest you get the FlashROM first, and after that you can always look for an FM cart later.
This sounds like the way to go, thanks.
It seems to me that you are getting flooded with information here...
I will wrap up the sound issue a bit... MSX has tons of sound extensions, but between the lines I read that you seem to be mostly in to games, so that can be used to narrow the list down quite a much:
- MSX-MUSiC (OPLL) This is THE extension that you want. If you are a handyman, good tip is that Alexey sells DIY version of MSX-MUSIC for a very user friendly price that you can put between Z80 and board... How ever it requires some soldering & knowledge of MSX internals. See here for details: https://www.msx.org/forum/msx-talk/hardware/f4fmpac-light-co...
- MSX-AUDIO (OPL1) Alhough this was the first and official music extension it is rarely supported... Even in these cases the game will also work with MSX-MUSIC in 99% of times. It may make music experience better but it is definitely not a must
- Konami SCC... In games this is practically used only by Konami. Yes, there are some exceptions, but they are more just curiosity. The importance of this chip comes from the fact that Konami was THE company that everyone in MSX scene remembers. Number of games that support this chip is not that huge, but they are all high quality games.
- Moonsound (OPL4) This is loved by musicians in Europe and therefore it has made it's way also to some modern games. How ever the number of game titles is not big for this either. Number of music disks is much greater than number of games.
There are also tons of other music extensions from Yamaha SFG-05 to C64 SID in Playsoniq, but these have no value to gamers at all.
Currently best option for storage is indeed MegaflashROM + SCC + SD. I would also like to recommend Nowind that is an USB <-> MSX storage adapter for playing games from PC HDD, but currently it is not available for sale.
What comes to joysticks... Yes you are right... MSX has it's own pinout, but luckily it is compatible enough with Atari so that Atari joysticks can be used as 1-button joysticks on MSX. The other button goes to pin 7. Pin 5 is +5v. With mice the situation is a bit harder. You need MSX specific mouse, but that is also something that is also not important for gaming purposes.
And then finally few words about memory... If you play Japanese games from disk then 64KB might be enough. If you plan to use Mega flash ROM or you want to play European games then 128KB is minimum requirement. With 512KB you can do practically everything. Anything beyond that is not important for any gaming purposes.
Thanks for the info NYYRIKKI, don't worry about me getting flooded with info, I'm like a sponge - I just want to soak it all up
That Z80 plug-in FM extension board is really neat, unfortunately I only have basic soldering equipment not SMD, does anyone sell a pre-populated board?
Well SCC can be added via SCC Konami carts, memory upgrades can also be added to machines (Some Sony machines already have the mapper chip) and FM pak can be added separately as a cart or add-on board, so adding an ide compatible SD card solution can now be added cheaply? This is what in a way gets my goat about the Megaflashrom cart - you are paying more for the "all in one" convenience, except that it lacks FM pak sound so doesn't quite cover all bases, so you still need to add a seperate FM pak solution.
This is what I have gathered from my very short time as a new MSX user, correct me if I am wrong. (or hurry up and develop a cheaper real all-in-one solution 
)