Melman this may be teaching granny about eggs, but- check your melodeon is in tune especially if there is another fixed reed instrument.
Is there another instrument playing base - if so - no need to use ( or amplify )your base. Most accordians have a much better range of bases, as do base guitars.
One melodeon is about the equivalent of 4 fiddles. Do you need to amplify at all in medium sized venues?
Are you going to go through the bands sound system - or do they expect you to have your own amp as well? Do they need you for the base or the melody or both? One or 2 pick ups needed.
Make sure that you can hear the lead instrument - you need someone to set the pace, speed up / slow down/ finish for the dancers with the rest of the band following.
A mike + stand means you can use it for other things. Do you also sing/call? A mike is easier if you change instruments - but takes up more room in the car.
I like to stand during a barn dances so I use one mike ( my old trusted Sures )at about waist level so I can turn a bit left or right if I want more base or melody - depending on what the other instruments are doing. I have another mike for the calling.
+ your purchases are tax deductables against your earnings.
Enjoy the playing.
FloraG.