Can't really argue with those prices, just DIY is where the savings would be, plus if plaster-boarding anyway, buying thinner ( cheaper ) frame material and retro-fitting with insulated PB would ba another way of killing two birds with one stone. However, just buying one increment up on those SIPs prices to achieve the same result and fitting PB to those ( prob over a service counter batten to allow for electrics etc ) seems a more straightforward direction.
Buy insulation once, reduce heating bills forever I'd still be and advocate of A/C though, for both heating ( which should be negligible in this type of structure ) but more importantly not to cook to death in the summer ( a surprising amount of nuisance noise comes into my 'office' from just cracking open the moderate sized window ( which I have to shut to hold a business conversation ) so be mindful you may not always be able to keep the sliders open all of the time )), but I don't yet have the acoustic batts in the roof void which may help there too.
I ran Cat 6 out there too, 1x for external WiFi AP which I mounted on the external 'office' wall ( so the garden now benefits from a very robust WiFi signal ), 1x for the desktop, and 1x for the network / internet printer. You could just get a cable from your existing BB router and take it to an external AP ( like a Ruckus heavy traffic unit ) and hook everything up to that. Depends on your dependency, but a hard wired cable from the desktop is way better than wireless.