AT the end of the day you need to do what you feel comfortable with. But realize that if you offer a crap product, you will get crap results. I'm not saying that not updating means it's crap, however I think things should be clean, safe and operational. You are going to want to update things at some point. I would think about it as sort of a property reset. There is always a risk of tenants destroying things, it's the nature of this business. Don't let that scare you. The house will probably be a disaster, they'll likely treat it like crap after they find out you are not renewing their lease, be prepared, try not to get emmotional about it. They may answer the rehab question for you. you may have to do major re-hab.
Random thoughts....
Don't over-improve
A few updates can go a long way to finding better quality tenants
Putting money into your property is the cost of doing business.
What is your competitions product? Are they renting out houses from the 70's as well? Are they updated and getting higher rent?
Like I said, you know your market better than anyone.