You state 20mm roundbar, that is way over the norm for domestic products, 12mm to 16mm being the most common, or are you intending to enter the top end of the market?
If you were making a gate then yes the 185 would weld the bar into the frame sufficiently to hold it.
That does not mean the machine has 16mm thickness welding capabilty, on flat sheet for example.
It would though be using your machine at full whack and no matter how good a MIG is, it does not do them any good working flat out all the time.
When making gates I tended to run my 235 at roughly 60% to 80% of its power rating.
If you buy the 185, I will guarantee 100% within 6 months you will wish you had bought the 235, for gates etc extra power is always good to have, it's best to buy once and buy right.
The link is from the guy a lot of members here have bought the Portamig from, I doubt you will find them cheaper.
There are other machines, Oxfords etc that are made to the same standard and a few quid saving may be possible if you search around