There is no question that insulating the walls would be a good idea. Just how much would depend on the climate area where your are, but insulation will reduce heat loss in the winter and help keep you house cooler in the summer. Insulation is one job where it really pays to have the pros do it. Several years ago I built a large garage/shop. I insulated the walls myself before I drywalled, but I wanted to blow insulation into the attic. I found that the local insulation company would supply the insulation and two men to handle the blower and hose for the same price as Home Depot's cost for the insulation and free use of their blower. I would still have to have an assistant and do the work. They can also advise you one what needs to be done to the soffits and likely do what needs to be done for a surprisingly low cost.