Do you still have that 8-gallon canning pot? Is it still in good shape? If so, you could use a picnic cooler as a mash tun. That will work fine for stepped infusions using boiling water. You can also use the 8-gallon pot for decoctions by pulling them from the mash tun, heating to rest and boil, then returning to the main mash. Then, I'd suggest you go with at least a 10-gallon pot for your boil kettle and, as others have said, you'll probably want a 15-gallon boil kettle if you're going to do any 10-gallon batches. Either the 8-gallon canner or a new pot could do double duty as your hot liquor tank.
My boil kettle is a converted Sanke keg (15.5 gallons) with a valve and thermometer. I like not having to siphon, and the thermometer is helpful for knowing when the wort is first approaching the boil and for watching temperature while cooling the wort with my immersion chiller. Those Boilermaker kettles are very nice if you can afford them, and I think one would be great as a boil kettle. However, unless you're planning to do direct-fired heating of your main mash tun, I think a cooler is every bit as good and a lot less expensive. The money you could save on the mash tun would go a long way toward setting up good temperature-controlled fermentation.