You could have a difficult time with the purchase of a business that much larger than your own. Consider that most businesses purchase a smaller company which they integrate into their existing way of doing business. Unless you have a very strong personality, you may find yourself trying to adapt your business to someone else's way of doing things. I am not saying you can't/shouldn't do it, just be ready for a challenge.
The biggest factor I would consider in your position would be how much the current owner is willing to help you achieve a smooth transition.
Do you have a strong enough relationship with him, that he would be truly interested in your success?
Is he willing to stay on as a salesman/manager for a few years and slowly transition his business to you?
Is he planning on financing the purchase for you so that you do not start out on your own with a huge burden of debt?