when you talk about basic MIDI and old time Midi Accordions,
i personally found that using multiple sound generators was
the easier and better approach
old time synths typically did something great, and most other stuff
just OK, so i had one unit for Organs/drawbars, another for Strings,
something else for General Midi, something for lead lines, and
something else for Bass
this allowed me to tweak the best out of units that were strong to
begin with in certain categories of sound, and also to have multiple
Volume Pedals (each audio feed through pedals into mixer)
in general, it is hard to beat a simple Roland Sound Canvas for
the basic GM sounds, (SC-155 is the handiest) and most any Korg
like an O or X is easy to tweak lead sounds out of.. there were ORLA
and several other brands for Drawbars, even new on Amazon ferrofish
drawbars are only $400, and the Waldorf Strings are the same price range..
all those are 1/2 rack size. Some of the newer era sound modules like
the Roland Sonic Cell also can playback some types of Media files as well
as having top notch Piano's and General MIDI soundsets
and rack stuff can be rolled to a gig
lots of the people here know how to do GM soundsourcing
from simple basic computers too with amazing sound quality..
some great Virtual Soundsets are shareware or public domain
for the MIDI accordion, while plenty of bargains are out there,
it is very hard to beat a used Roland FR3 or FR2 under $1500
usually as a basic MIDI controller, and with velocity keys, which
you will not have on an older MIDI accordion in similar price range,
and with a V-accordion you can just use the internal generated
left hand sounds (Bass, Tuba, etc.) and that is 1 less synth in the rack
the roland XV 5050 you mention i think also has a usb port and
can be used directly with a computer as a sound module/source ?
which might be handy, whereas the older synth modules need a MIDI
connection to interface with a computer
so a lot of this will depend on what sound families you want to have
and have control over, and if you want to blend multiple sound sources
in real time or if you just want one sound at a time for fun