A keg is the way to go. I converted fifteen years ago because I wanted to step mash and have never regretted it. You'll have so much more versatility in your temperature control. You'll be able to mash up to 25 lbs of grain and, if your boiler is big enough, do 10 gallon batches (I use a keg for boiling and HLT as well) It's twice the beer for the same amount (well almost) of work. You'll be stoked!
As far as insulating it for the mash, I just squirt the metal skirt at the bottom and throw a sleeping bag over the whole thing, burner and all. I never lose more than a degree or two in a 90 minute mash.
You should definately fit your keg with a drain and a manifold, though, for sparging. It's easy and well worth the effort.
cheers, -p