I don't particularly like the NA M111 engines, they are course and gruff, not refined and smooth like the older four cylinder M102s or the sixes. They also need revving quite hard to make everyday progress which makes them frustrating to drive.
A W124 cabriolet with a 220 IMHO isn't quite going to offer the 'experience' associated with a luxury four-seater cabriolet. It's heavier than the coupé/saloon due to the extra bracing and roof mechanism. You need more torque.
The 320 is worth waiting for - as said, not a great deal more expensive to run, likely to be better specced and far more enjoyable to own
As with many cars like this - you do get what you pay for do a degree, but don't assume that higher mileage/tatty/4-cylinder engined cars are worth almost the same as nicely cared for well-specced 320s just because they look the same!
Look out for a well cared for car at sensible money - you could easily spend a couple of thousand or more bringing a slightly tatty one up to spec - loom, HG, wheel refurb/tyres, rusty wings, roof repairs, servicing work etc.
Coming into the more expensive time of year for cabriolets so you might need to be patient
All IMHO of course
Will