A united Mongolia can only benefit from having increased trade with its two closest, and largest, neighbors. Peace among Inner and Outer Mongolia can only serve to help build trust with China and Russia to benefit the people in that region with increased economic trade. A united Mongolia can also benefit from more tourists in the region to bolster a sagging economy.
Mongolia is unified, and has been for many years - unless you count Mongolia as the entire area of ancient Mongolian conquest, in which case half of Europe would need to under Mongolian rule. Mongolia isn't a very developed country, and it most likely couldn't handle a drastic expansion of authority.