Basically you want to clean out every bit of ash you can. Us a stiff brush, power wire brush, vacuum cleaner, and scrapers to get the inside as close to bare metal as you can.
Then use a sponge and wash down as much of the inside that you can. I'd remove the chimney pipes and elbows, clean and wash out with the baking soda solution as best as you can. Then don't reinstall them, instead stuff the pipes with wadded up newspaper and wrap in newspaper. The paper will help absorb humidity and keep the metal as dry as possible.
After the inside of the stove has dried, then spray it down with LPS or other preservative spray. Or you could brush it down with light oil. What you are trying to do is seal the metal surface from exposure to oxygen and humidity. I'd stuff more wadded newspaper in the stove and especially in the flue exit.
Inside of the hopper will probably be rusted, wirebrush and spray paint with good enamel paint.
The stoker unit should be removed, all the ash, stray coal needs to be cleaned out from around the stoker mounting area, inside the stoker, and especially under the grate. Under the grate will usually be full of sand-like coal fines. The inside of the motor and gearbox should just need a blast of air or a good dusting with a vacuum. If the motor has oil point give then a drop or two of oil.
If your chimney is stainless steel, clean the ash out of the chimney and stuff the end of the pipe with newspaper, if you can access the top of the chimney do the same and cover it with plastic. If your chimney is a ceramic/masonry one, then the cleaning is not all that important, the ceramic liner is pretty much impervious to the acids in the ash.
I think any of the above efforts will help preserve the stove for many decades of use.
Hope this helps. Greg L