I don't think the cranes quite big enough either tbh but thought it would be a good example and tbh thought with gus's skills building one wouldn't be out of the question
Even starting with a brand new ifor cranes that size arnt expensive second hand often less than a grand for something that needs some work or there is this
https://www.ebay.co.uk/itm/223931990080. That looks ready to go
Plus the petrol powerpack parts come in less than the 7.5 tonners above
And the labour to do the work would be
outweighed by the running cost of the lorry so probably only a year or 2 before building the trailer becomes the cheaper option
by the time you spend a large wedge on the Ifor and find a decent Hiab to suit, which'll not be cheap (that example's start bid is £3.5k ) your're gonna be well up in the wonga stakes .... could run a 'cheapo' , older 7.5 for a long time, for the cost of putting a trailer together, which'd always be a compromise
AND you'd have to keep hold of the damned thing ... as said major pikie magnet
I'd rather be trundling around in a comfy 7.5, kitted for the task, when I needed it, than wrestling a damned great Ifor/crane combi behind me van ... would also be a pretty fair DVSA/plod magnet too, I'd imagine.

with the mileage / usage Gus'd give the truck, he'd also probably be able to negotiate a much more attractive maintenance/inspection regime than the usual six weeklies, for a truck doing hundreds/thousands of miles between six weeklies .... mine was on 4 monthly .... did very limited local mileage and used once or twice a week

... much like Gus's planned usage