The carpet may have to be replaced every time tenants leave.

Is there a possibility of just replacing the line up to the neighbor's connection to the lateral? 

I had to have a structural engineer come in and make recommendations. 

Age, pumping schedule and possible break down and replacement are all issues. 

Most of that time was as a sideline to my engineering job.  

This way I could keep the unit filled at the same time find a replacement. 

I am looking to replace with a hard flooring.

From some articles I have read though it sounds like exterior upgrades tend to have pretty good ROI, replacing the front door with a nice new steel one in a color that works well with the house, and new siding (or stucco or brick treatment) can be quite a good improvement in the look, that usually has pretty good ROI.

One is a townhouse with 3 bedrooms, 3.5 baths, built in 1992, around 1430 soft, purchase price is probably around $210000, rent is around $1800/month, needs to replace the roof which may cost $5000.

Last word of advice (and I apologize for the lengthy rant) but probably the most important thing I have learned about real estate investing is to be a "transaction engineer".