If you could sketch out a floorplan you would get a lot more feedback.

get your budget nailed down. then I would contact a local cabinet maker to come and estimate cabinetry which uses all the spaces without altering things. While you refrain from altering the space, much can be achieved with glass doored cabinets and other custom touches. But you need some counter run, without cabinets to counter, good base drawers, and wall cabinets to maximize storage.

You may be able to create an "L" with the sink wall and radiator wall. Please sketch your floor plan indicating all doorways, windows, and include adjacent rooms.

Hard decisions and lots of creativity in old houses. Those look like the original dish cabinets with glass fronts. Would you like to keep them? (I think they are beautiful)

Consider finding a way to unclutter your kitchen ... perhaps by adding storage in the form of a new corner pantry (repurpose your current cabinet to another room) -- something like this to make use of every possible inch of wall between your sink base cabinet and radiator -- but your own wall to wall and floor to ceiling shelving could be deeper and you could add bi-fold doors.

https://www.kissingrockkitchens.com/kitchen-corner-pantry-cabinet/kitchen-corner-pantry-cabinet-beautiful-tall-corner-kitchen-pantry-cabinet-how-to-build-a-in-the-walk/

https://www.houzz.com/photos/corner-pantry-traditional-kitchen-phvw-vp~4426550

http://rtscript.com/corner-kitchen-pantry-cabinet/aura/d780/pantry-cabinet-kitchen-cabinets-corner-pantry-with-corner.html

What is the measurement between your sink base cabinet and the corner and between the corner and a point about six inches from your radiator? You'd probably want to extend the corner pantry the same distance along each wall.

You also appear to have some under stair storage with metal shelving but there may be ways to get more storage from that space.

You could leave the current opening and just add drawers to the side.

https://www.pinterest.com/pin/386887424217427550/

or you might consider creating a cabinet with wheels that will roll beneath the stairs -- a set of drawers shaped to utilize as much of the under stair space as possible.

https://www.ikea.com/us/en/catalog/products/S59033838/

If the storage has wheels, you could park two cabinets beneath the stairs -- one lower to fit beneath at least part of the stairs and another, taller cabinet in front of that.

Would leave the window over the sink looking out where it is but the other two interior "windows" could become cabinets within the wall.