It has eyelets and brazeons, so just about any rack should fit it well. No need for a seatpost mounted rack with that bike. Will you be using panniers or just strapping on a milk crate occasionally or semi-permanently? Based on your stated uses, the weight capacity of the rack should not be an issue. Though I prefer beefy racks because you never know what you might try hauling and I don't like feeling the load wiggling around back there.

Using the rack as a fender can sort of work if you are trying to avoid the skunk stripe on your back. It obviously needs to have a solid deck, the longer the better. That is only one benefit of a rear fender though. A true rear fender with adequate coverage will do a good job at keeping road crap from getting thrown onto your drivetrain. That is something a rack is not going to prevent.