Coarse wet and dry should sort it out but do it with the iron and cap iron in and locked down but retracted below the mouth.
Use a sheet of float glass...or some marble or granite off-cuts...anything flat really and stick the wet and dry paper down on this with water. Then I usually make a fence out of a long piece of wood and run the side of the plane up against this to give a more pleasing look to the flattened surface.
The thing to look out for is that the front, rear and around the mouth are all on the same level. Ideally the bits in between would be lower or the same...not raised. You need to remove any raised areas caused by poor initial machining or subsequent corrosion
A perfectly flat sole along the entire length is not necessary and indeed...Japanese planes are deliberately made lower to reduce friction.
You don't need a polished surface...just a clean and smooth surface. Do the same for the sides if you want cosmetic improvements..but there is no real need as you probably won't be using this on a shooting board.
You couldn't get any more poorly maintained as this bootfair find from a while ago...
You don't have to go for a full restoration but it's fun doing it...
Some pictures would be helpful....
There are plenty of restoration threads on the UKW...just use the search engine with key words like "restoration" or "restore" etc.
Cheers
Jimi