Good measurement tools
Good drill index with quality bits
Good gunsmith screwdrivers (the more the merrier)
Good bench block
Good set of pin punches
Quality work mat
Cleaning solvents and lubes
Anything else would be gun/caliber specific and if you got everything you might encounter, you'd be broke before you got started. Smithy.
Salvation is through our Lord and Savior Jesus Christ and in Him alone.