Hardwood floors for newbie DIYer.

[Paul Winka]

A rehab is nearly done and the occupancy permit will be granted shortly afterward. I still have to take care of the hardwood floors and paint after moving in. The painting shouldn't be too bad, but about those floors, I need to sand, stain, and put down polyurethane. I am contemplating doing that myself. They are somewhat water damaged but salvageable, see pic. I plan on staining a darker color. 

For those first-time DIYers that did their own floors, was it as tough as you expected or did you mess your floors up? What's the trickiest part of getting right and any other tips? What grit of paper did you use on each stage of the drum sanding? Thanks!