You may also find airlines pay a retainer for handling at possible en-route diversions. They may also locate towbars or towing head adaptors to cater for specific aircraft. But you don't really need that much equipment. Most airports can handle one or two aircraft on self-manoevering stands. Specialist equipment such as GPU's, toilet trucks, water trucks etc. all have standards fittings which leaves boarding stairs. You just need one set adjustable ones.
Emergency towing can be done from the main gear if you don't have a TBL tug. But failing that you shove it off the runway with a bulldozer if you really have to.