If you were good at DIY you’d have a few more options. I’m currently working from ‘the shed’. Ours was a self assembly summer house – only overlap panels and fairly cheap. assembled it myself with a decked patio and raised planters, but then set about installing insulation (used the laminated bubble wrap type material, then ply lined the whole thing. Installed electrics and lights myself using armoured cable, all painted and decorated inside. All that more than tripled the cost of the shed.
It’s only 6×8 and was really something for my wife to do her crafts in, but it’s become an office. We don’t secure it as it’s not that secure – front is mainly perspex and two doors – easy to kick in, so laptop and screen comes out every night.
For your budget you’ll get something very nice, but there may be extra work to insulate it as most are single walled. Even the ‘basic’ insulation works exceptionally well – stops the heat build up in summer – it’s quite pleasant working in there considering how hot an un-insulated shed gets in direct sun.
Looks like I’ll be here for another 6 months.