I wouldn't recommend starting out with a massive inventory of strings - start with a good syn.gut, a good poly and a good multi and offer the rest as a special order - that way you'll save yourself massive expense for a venture that may not work and still offer a good service. If you find after a while that you keep having to order a certain string in then you can stock it full time.
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)