If you have a good relationship with the people you bought the place from, talk to them once everything is fixed and see if you can work something out with them. If not, you'll have an easy case in small claims court, which is a simple and inexpensive process in most areas, without a lawyer.
If your plumbing was working before the septic backed up, you shouldn't need to have it replaced. If you choose to have it replaced, keep the old cast iron pipes, or have whoever does the work give you a discount to take them... they are worth decent money at a scrap yard.
You've already dealt with pretty much the worst you can experience when doing plumbing work. It's an easy skill to learn that can save you (as you've seen) huge amounts of money!