I think it's a better idea to buy your land, have a new building put up, designed how you want it, and yes make it about 3 or 4 times larger than you have now.
If you plan and design the layout correctly, you can rent out some of the extra space to help pay for the larger building, then in the future when you need the additional space, you don't renew the lease on the extra space and you move into the other space.
You could build a 15K s/f building, design it as three separate store fronts, with partition walls in between, you occupy two of the spaces for now (10K s/f), rent the third 5K s/f space to someone else, to help you pay for the building.
In the future if something changes in your life, or later when you retire, you can rent out the three separate 5K s/f store fronts, and just be a landlord, like the guy making a living off of you now.
http://www.metalbuildingshopper.com/size_price.htm
The best part about the metal buildings is the metal frame is self supporting and the inside is a large open space (see photo below) which can be divided with non load-bearing partition walls that can be re-arranged as necessary for future modifications etc.
Additional photos show the type of building I like, it has a FLAT shed type roof structure and allows for a flat fascia area on the front of the building for signs and graphics as well as veneer brick to be applied to the front only.