If you dally too long on conquering your neighbours you may face diplomatic ties with England and the other territories, and England could potentially come to their rescue if called. It's also wise in this period to improve relations with England. Be careful not to be dragged into France's war with England, though, as you won't be ready to take on the juggernaut just yet. Scotland and the collective provinces of France (including France itself) make fine targets for your overtures, as do the provinces of Spain to the south.
Feel free to build more armies as necessary the last thing you want is to fail and have your opponent begin moving steadily past their starting three armies in retaliation.
Don't worry about army morale or losing a bit of money at first your goal is to crush neighbouring governments and install your own.
If you are responsible for one of the above issues.