google sewing room ideas and see what others have done.
I took the doors off my closet in the sewing room and have bookshelves and plastic cabinets.

In working with upholstery you will need a fairly large work area around your machine. That's why I would suggest a sturdy table for your machine that could be on casters and rolled out of your closet when being used and rolled back inside when you are through. A shelf on the bottom of one side might be the place to store your Singer. Remember, of course, that it has to fit in the closet with the doors shut so the measurements will have to be right. Lots of shelves overhead would be nice for storing your supplies. Think about transparent storage bins for keeping all the loose stuff. Keep in mind the higher you go with shelves the lighter the weight of containers. (I found that out the hard way) Also organization is key. Label your containers and keep like items together to avoid plundering through things to find something. Don't forget the back of the doors where you could mount a bulletin board for notes and ideas. (If the doors are the swing out kind) Have fun.after-all it's whatever works best for you!

If it's a beast, make a roll-around cart for it that can roll up to your work table so you aren't lifting it.

Look at the way good workshops have the proper height work bench, and small carts that roll into position with the tool permanently mounted.

A place for hanging material, on rods for heavy stuff and hangers for dress weight. Or folded on shelves.
A place for your iron and board.

A corkboard for pinning up pattern instruction sheets and apttern pieces as you use them (my mom'strick).

As many shelves as you can fit in there, sized for some cheap brand of plastic storage: shoebox size and maybe a bit larger for the notions and threads. Take a cone with you when you buy boxes.