Does your Massey 135 have draft control? (I don't know spec's of M135s). If not, a depth control wheel on plough would be a good thing.
Also thinking that, for the odd bit of plot ploughing, a basic single-furrow plough shouldn't be sniffed at if the price is right.
I've been re-considering recently whether I need a small plough (or not) for much the same purpose as you Justin. Undecided, but I've been looking. Quite a few used single/2-furrow ploughs available (web searches), but not available anywhere near my neck of the woods. So I've looked at new offerings: I cannot vouch for, but machinerysales EU offerings (single or 2-furrow versions) are a very decent price and, from my untrained eye, probably quite good enough for most "small-holder" needs. (They could well be better than that, but I wouldn't know - what I do know is that I wouldn't want to pay more - new or used - for something I wouldn't use very often.)