I am going to be redoing my kitchen in a couple weeks (walls, cabinets, floors, etc). I was wondering if it would make sense to replace the sink while I was at it? The sink now is not in bad condition, just some stains around the drain. However, since I am going to have everything out anyway, does it make sense just to replace the sink as well? I have some extra money left in my budget for the kitchen that is more than enough to pay for the sink.

Also, for the counter top, I was thinking about just creating a tile counter top that also goes up the wall to where the wall cabinets end. Could I just cut a piece of durock, attach it to the base cabinet, and attach the tiles to this? In addition, would it also make sense to use durock for the wall where I would attach the tiles?

In my opinion, if you are doing the rest of the kitchen anyway, might as well get the sink in there. The rule "everything but the kitchen sink" doesn't apply here. ;)

As for the counters, I'm not an expert but I believe you need plywood under the durock.