Notice: Pretty much all 27 countries with higher defense budgets as a percentage of GDP are either not all that large, or do not have a strong economy. China has the world’s second largest economy, but that’s with over a billion people. We have the world’s largest economy (~15 Trillion GDP out of global GDP of ~60 Trillion), so we can spend much less as a percentage of our GDP and still have a large, effective military force.
No we can't. We do R&D, most of the small nations do not. We pay our troops well, they mostly draft theirs and pay them very little.