It also really depends on the collector, while I agree that from a momentary perspective, those mowers hit the mark, and I certainly cant say I don't want them, for me, my heart lies elsewhere.
From the late 60's anything with a G2 or 3Q carby is quite a find, these carbies had a very short, intermittent run.
I'm also a sucker for Series 70MKII and MKIII engines, with the G3 carby.
And then there's the 3FI carby, I have diagrams of it, and references to it, but I have no idea what it was released on, and how many there are.
The 3FI carbies is the industrial version of the 3F/3L carbies (the first victa carbies) but it has a variable jet, based of a needle attached to the bottom of the throttle slider, and a few other modifications.
I know of more people with fan mowers, or brass nut carbies, than I do people with anything that came with a 3F, I'd love to get my hands on something with that.
Also there's certain variations that spring to mind as hard to find, the 2nd and 3rd VC-125 are tricky finds, as is the original 'green top' VC-160 Auto Drive, I love the 63-64 corvette, and the 64 Cosul 2 with it's side catcher.
Going outside of Victa get's hard yes, it's a brand to brand thing, early scott bonnar rotaries are pretty cool (J.A.P Engines) or early simplex powered rovers, there's also an interesting one on ebay at he moment, a mowfast, quite an interesting looking set up.
The there is reel mowers, which go much further back, and age can play a great factor in value, as can location, in the UK there is not really a big rotary collector scene, but the passion and prices you see for some of those early reel mowers is amazing.
Personally I'd love an early Atco Standard, or a nice Scotty 45 from the reel side of things.
If you are looking to crack into collecting, I would suggest looking for something that grabs you, either something that you can link to a memory, or something you just plain like.
If your looking to increase the Value, and Standing of your collection, definitely keep your eyes out for those early Victa's, like the predicta, personally, I love the look of the 2a.
Outside Victa, just look for something 1955-1965 that you have not seen around much, I can put up some photo's of my early non victa's if you like, to give you an Idea.