I guess that the first 'rule' to follow would be, allow for growth and change. That being said, a 4x5 enlarger may be a better choice. You may want to go to the older, slower, larger format as, like me, you get older, slower, larger !
Even illumination is a possible problem on some MF enlargers at the extremes. I recall, years past, using a 23C for MF, 6X6 seemed fine, but 6X7 did have minor edge fall-off. Since I typically burned the edges somewhat, this was not a major problem.
Generally, the LF enlargers are built to a more 'professional standard', while many SF/MF are more 'mass market hobbyist' targeted. There are great buys everywhere on Omegas, Beselers, LPL and even Durst. (Let's not forget DeVere across the pond!) At the price of used enlargers today, go large, and don't look back.