We have one of those in our garden. They're big enough to move about in
if you crouch, but I think you'd be hard pressed to spray something
bigger than a breadbin in it.
If I needed to spray something properly, it'd be a matter of hanging it
on wire from a fixing and moving around it, spraying it from the proper
distance with sufficient lighting to see coating. That little shed isn't
going to be big enough to get proper spray-gun distances inside, let
alone move around without knocking or moving the suspended object.
There's also not enough room for lighting and the doors will not stop
finish-ruining dust getting inside. The ceiling is also very flimsy,
unable to take any load and will start to let rainwater in if dented
even slightly. Putting a suspending frame inside will further reduce the
free manoeuvring space.
A spray-booth needs ventilation, a dust-free environment, shadow-free
lighting, and hangers capable of a reasonable load.
Sharing the cabin between spray and just about anything else is also not
a good idea to my mind. No matter how well ventilated it is, paint
particles will migrate into and onto anything else stored in there, and
the place will stink.
For £100, we could build a more suitable spray-booth from scratch out of
a few OSB sheets and some scrap wood. Just a square wooden hut with a
ceiling-hook, a HEPA-filtered positive-pressure fan and a breathing snorkel.