You also Mentioned that your septic is as old as the home itself? I certainly hope this isnt the case. I know here in friged Wisconsin, there are strict laws regulating of Septic Sytems in rural comunities. If it is that old im almost certain that there is some kind of leakage going on as well, and believe me you do not want that to happen. We certainly dont want our you know what in our streams and underground water supplies. Replacing the system can be a big job and big bucks, contractors will more then likely charge tons for this job. They say its the labor costs, i beg the differ its the equipment they will use to do the job, your talking somehwere around the lines of $50+ hourly just haven the equipment there. And again if weather isnt nice were talking longer time frames and bigger bucks. Harold is right your better off Building from scratch then tackling this and many other changes you might have in mind.
Theres one thing you should consider when buying this old fixer upper and thats, why am i interested in this home? Is it simple because its cheaper to obtain, at first, but as you go on with all these renovations will it be worth it?
Calculate it in terms as to will the value increase or will it simple stay the same due to fact all your doing is maint. It may be a new wall or a new fauccet but lets be real the home value wont rise that much. I own an old home myself with an abstract title dateing back as far as 1845. I thought about doing similar things to the home as you are now, but the reality is you can't turn an old home into a palace without haven big bucks. I found it more satisfing to not demo the inside of the home but restore it to its original design, to me theres alot of history in these kind of homes and it would be a shame to clash to much modern upgrades with old fasion look. Old homes have class and a sense of Character. Try to maintian that look you might find it more gratifying.