Take a tape measure and measure every possible tight spot to make sure you will fit. As mentioned above removing the head may have to occur, don't trust the owner to remember if the took it off to bring it in. I moved a pin once and it would not fit through any opening. The owner than mentioned, "oh yeah, we did remodel the whole house once that pin was already inside".

Try to find a stair climbing dolly to rent or just try to find in the classified ads some home type movers/general handymen. Two reasonably strong people can hump a machine up the stairs for you on a dolly without any trouble and would likely be much cheaper and quicker compared to you renting a stair climbing dolly or hiring a moving company. The problem is finding two people to hire and help you.

As a last ditch you can always just break the pin all the way down and easily move the piece by yourself, but it will take time.
1-Remove the head
2-Remove the playfield (and keep out the playfield glass)
3-Remove the legs

The cabinet is light without the playfield or glass installed. Easy to move up the stairs with a dolly. Just put the playfield back into the cabinet to transport, otherwise you are guaranteed to mess up parts on the playfield in transport.

Best of luck, you will get very good at moving pins in time. If you do it much pin moving a powered climbing dolly and lift table will quickly become "must haves".