New window into old stone wall

I'm working on my planning application for converting an old stone building. Its Victorian, the walls are about 600mm thick. Fairly neatly laid on the outside rubble in the middle.

To get a nice layout it'll be much better if I can get a window in the gable end. I'm trying to gauge how hard this will be before committing to it. Any advice on the process or experience of similar would be appreciated. I'm not looking to go too wide. Probably around 600mm wide to match the opening above, but bigger might be better if it doesn't massively increase the job and risk. Just to add, there is a lean-to shed which will make propping a bit trickier but that will be coming down (and later rebuilt)

Some pictures of the wall, one with a Photoshop impression of the window:

Why not approach the job by removing the lean-to shed, and then scaffolding up to work from the outside and the inside of the gable - you could attend to both openings at the same time.
A mason experienced in stone could open up and make good the surround in stone rather than brick.
From a structural point of view there's no problem so long as the outer and inner lintels are done separately before opening up the hole.

Are you doing it DIY, or getting a builder in? Random rubble stonework isn't as stable as most brickwork so a bit more care is usually required.
That's quite a common form of construction where I live, so I have done a few jobs like that. Here's a pic of one I've got to repoint in lime.

Thanks. I am considering DIY, as I would for most jobs. I'm still trying to understand the process. For a brick wall the lintel would go in then the opening gets cut under it but the brickwork that supports the lintel is already there. In this case the brickwork which will support the lintel doesn't exist. Just trying to understand what gets supported where and the order things happen., and also the risks.

That's the problem with a random rubble wall. You need to get plenty of needles through the wall, above where the lintels will go.
Then remove enough stone to build up the jambs. Then the lintels go in and fill up between the needles. When safe remove the needles and fill in the rest.
I normally use LBC commons for filling in the centre of the wall, as they are much easier to cut to the required shape, and face the exterior with stone.