Be careful with cost basis for capital gain/loss purposes as that could be different from the cost basis used for depreciation.

Improvements should be included but those should have already been listed as separate assets on prior year returns for depreciation purposes (or included as part of the property's cost basis if those were made prior to the property being placed in service).  Not sure if I understand your statement "since there is no value assigned to them".

Major repairs are maintenance items that generally do not extend the life of the asset and should not be considered part of the cost basis.

The adjusted cost basis would be as you described sans major repairs.  Depreciation allowed or allowable, whichever is larger, would be subject to §1250 recapture to the extent there is any gain.