I'll leave the first question to a better expert but I will note that the Qin army didn't simply unite "China" (which at the time was only the area between the Ordos Bend and Hangzhou Bay): they went on to expand the entire way through Hubei, Hunan, and Guangdong while holding down rebellions and fending off the Xiongnu nomads.

The second question is simple, though: None. No one except the Xiongnu had any way of projecting power into China and, even if they were interested in a Hunnish invasion rather than border raiding (which they weren't), they would have had a long hard slog at it.

The Qin army was effective in its time. They had crossbows with about 100 arrows in a quiver and the heads of the arrows even had a higher tin content to make them sharper. The had chariots, spears, swords, and hooked instruments to counter swords or spears. We know this because in 1974 the terra cotta warriors were unearthed near Xi'an. The ~5000 soldiers each have individual faces and were painted. The army also had a new organization style. Soldiers were rewarded for bravery and merit instead of just given to nobles. Very few polities at the time could match China's size and organization. For a recent article see this one. Hope that helps