I think it really depends on what you buy the first time around, and how your business changes. All equipment has a service life, so it may be replaced over time due to wear. When I was DJing I was doing several hundred shows a year, so even with maintenance that puts a lot of wear on a loudspeaker. I kept corners, handles and wheels, crossover parts, diaphragms and other parts on hand to ensure that my rigs were in operable condition for every show.
Speaker purchases should carry the consideration of the types of shows you do, sizes of venues, transportation, and of course, budget. I list budget last because frankly, a deal is no deal at all if it doesn't suit the purpose. First and foremost we need to ensure good sound to our clients.
Ben