Thank you for the clarification. Yes, the damage on mine is almost certainly from a jack being used on the underside of the sill skin as opposed to the actual jacking point. It is pretty isolated and is just at the rear of the sill itself. I have someone coming to address it in a week or two along with some other little areas. Hoping to get them dealt with now so I don't have sill sections and arch replacements later.
I had seen cheaper versions of these lifts, the Hawk tools one is a fair bit cheaper and was probably one I'd go for. When it comes to the Chinese ones, I am always a bit wary of the quality, although I suspect most of these are made there anyway and rebadged locally.
That looks like a really useful way of elevating these however, dropping the entire frame out just seems to make so much sense given the space restrictions when in situ. I'm hopefully nowhere near having to do jobs this big yet but I do intend keeping mine for many, many years so it will happen at some point I'm sure.