to me thats got 'garage conversion' written all over it..
and what id do is 'board it out'..easier, quicker, cleaner, gives you the added benefit of being able to comply with current building regs by studding it and adding insulation (cellotex etc)...same with the floor (cellotex, 18/22mm chipboard)
plus id brick the door up (usually needs a footing) and put a window in the front..
dunno how far you wanna take it but if you dont wanna board it, bond it cos render wont stick...but if you bond it expect damp to come through cos its gonna be a single skin..stud it off and you avoid that problem...
if you wanna put 'fixings' in the wall, the way i'd do it is to work out where you wanna put your fixings and incorporate studwork before you board it...
do it the proper way and you need a building application (get a form from local council) costs a percentage of the job (tell em its 200 quid) youlll pay under 100 quid for them to inspect footing/insulation etc...
id do the electrics too...needs a spark with part p though...new ring innit...
easy job if you know wot your doin, i used to charge 4 grand with 2 double sockets, light pendant and a rad (taken off from house), finished in about 6 days...
do it on your own itll take longer but you pay about a grand maybe 1200 inc materials and sparky...