When I buy cards, can I use cards in my hand AND unit cards. Or do I need to choose? Unit used to buy cards, are they discarded in my discard pile?
From the "Buying Cards" section of the rules: "The twelve cards... are available to be purchased by discarding cards in the Player's hand for their Gold, or by spending Gold given by a Unit Card in their Legion that total up to the Cost of the card." The use of the word "or" is a bit confusing, but the word "total" would seem to indicate that you can combine these two sources of gold: (i) cards discarded from your hand that produce the amount of gold indicated on the gold icon, and (ii) Legion units that generate gold from their special text ability without being discarded. That's how I understand it, anyway. See this thread for more details from the designer.
When attacking I can play a spell card per unit. Can I count mercenaries played in the previous step?
From the rules: "There are three Unit types: Mercenaries, Armies, and Henchmen." Also: "Each unit in combat is allowed to cast one Spell..." Although there is a rule about Mercenaries not being able to have equipment attached to them, there is no rule about them not being able to cast spells.