Right way to include nusoap in D7
It depends what you need really.

If you only need nusoap once, for a single project, then just download the library and include it using include (or even better, module_load_include(), that way it's still a 'Drupal' method!) There's no point writing a whole new module just to include a few files.

If you think you can provide useful wrappers for nusoap functionality though (as in ones not intrinsically provided by the library itself), or if you're likely to want to use nusoap again in a future project, it will probably save you some time later on to write a new module.

If you did this then you also have the added benefit of being able to contribute the module back to the community; a lightweight Drupal-capable soap library would probably be useful to a lot of people.

If I could offer any advice it would be to plan out what functionality you'd want your new module to have (what entities it could interface with, view/rules integration etc) before you dive in and write it. It's a lot more difficult to make a module generic halfway through writing it than it is at the beginning :)