RE: Buffers vs retracting spool holders
marcovaldo,
agree with gnat that above versus below does not matter.
My MMU2 worked with neither buffer nor spool rewinder when I had the spools two feet (~60cm) behind the printer. The filament would just eject from the MMU into air (only had a small bit of PTFE at the MMU and spool holder) and would not kink or twist since the ejected length was small compared to the overall length between the spool and printer. But, that was a temporary placement to get the MMU and printer working. Once I had space constraints, I started looking into (and eventually working on) rewinders.
You have a lot of options if you can dedicate a second lack table to to solve this problem.
I think that solution Graham Law (@gandboss) came up with ( https://www.thingiverse.com/thing:3690758 ) is great and might work well for you. It basically uses old empty spools as "buffers" and would work well for a vertical setup like you are describing.
If you want to use less space, you cannot go wrong with a rewinder from either me, Martin, or Vincent.
good luck....