New TT or old

I bought a new cartridge for my old Thorens rather than buying a new TT, so I struggled with this question. I don't know the B&O, but I'd say go for the cartridge. If it doesn't work out, you still have the cartridge to put in a new TT. Good luck

Get a better table and you won't be restricted to B&O cartridges.

A B&O cartridge would not be a very good match with modern medium mass tonearms. It is VERY low mass and requires a low mass arm, not very common nowadays. So buying a cartridge( if they still make them) with the idea of passing it to another 'table if need be may not make so much sense.

I see nothing wrong with playing your LP's on a turntable of the same vintage as your vinyl. Do it, and enjoy it. If you decide to get more into it later, I'd recommend a record cleaning machine before upgrading the turntable. Just my two cents.