I suggest using Chemtech 1L Concrete And Driveway Cleaner. It is biodegradable and non-toxic and does not affect plastic, rubber, glass, and most metals. Please note that this is a concentrate and must be diluted in water. Use a stiff bristle brush broom to remove dirt and debris while using Chemtech.
We decided to hire someone professional who will use the pressure washer with degreaser. However, we enquired a few of these people about the pressure wash process and they all say even with pressure cleaners some stains will remain on the floor. - ivanptr
Some stains like paint and grease stains are very hard to remove. You will only be able to find out once you give it a very good clean. If after using the cleaning products there are still paint stains on the floor, you will have to use a steel scraper to remove them.
For heavy oil and grease stains, I suggest using CLR 750ml Oil And Grease Remover. With an older age of the floor, some stains will be difficult to remove. Once the floor is totally clean and dry, you can then see what is left and determine if you can paint over them.
With the majority of stains and marks removed and a pressure washer used, that would remove all loose dirt and debris. From there, I recommend either painting the floor or do an epoxy garage floor project. Either method will give you good results and a strong cover from future stains.