front end or swap

Do i neccesarily need a front end? Its o mutch more money to get a front end im not sure if its worth the money, and im on a tight buddget. If i get a BB enginge with a trany and it doesnt come with a ecu or a wiring harness will i need to buy these, or will they comply with my cars existing gear? And how mutch of the exhasut system will i have to replace, and how mutch will it cost.