It really does depend on how close you can get to the stones, how they're laid out, etc.
Over the years I've taken a few, and usually I wasn't paying much attention to bokeh etc since that wasn't my primary goal for the stones.
That said, one of my best shots was purely unplanned (I was taking it to share on Facebook for family), and it was done with the lowly F 35-70mm (stopped down, to boot). This was shot at 35mm as I was trying to catch the planters in the shot, and at f/6.3.
Last winter I prowled the cemetery with my 300mm when I saw the gates open and was having fun catching shots - but I of course had to keep some distance. Bear in mind the longer the lens, the more you're going to have to hope the light behaves itself.
If I had to pick one lens only, I'd probably go for a fastish midrange zoom (something in the 35-200 range). That way you can pick your location a LOT easier and not have to worry about standing on someone's plot to get your shots in. This has two benefits - people tend to get cranky when you stand on Granddad's grave, and (especially with older cemeteries) you actually have a very good chance of having the unpleasant experience of having your foot go into the grave (or worse) if you do so.
Bear in mind you will probably be limited to shooting from the pathways.
Out of Pentax's current lineup, I would probably actually go for the 18-135mm if I had my choice, or the DA* 50-135mm if I had the budget to get it. If money wasn't an object, of course, I'm betting the D FA* 70-200mm would/will be the best Pentax lens for this type of thing.