Insulation everywhere is probably the most important, but make sure it’s breathable to stop rot (use a membrane). Then everything else like power, light, heating (I would suggest infra-red to warm her not the air), decent flooring – insulation and then waterproof t&G chipboard covered in something like laminate or vinyl. Sort the windows and doors out to with something better insulated and better sealed if not already. Phoneline and internet. A powerline adapter might work. You will need some armoured cable and ideally a separate CU unit for it unless you just plan to run it off some sort of extension lead which you can do yourself but will be limited on power. Should be fine for an laptop, printer etc but not for electric heaters really.
It may well be easier, if you have the room, to build a new office for her and keep the shed for your bikes though.