Based on my experience, ideally go with the 70 qt or larger cooler, a 20 gallon kettle for your boiler, and a 15 gallon kettle/keggle for your HLT. You can
do it with a 15 gallon boiler and a 10 gallon HLT, but you're pushing the limits, particularly on the 15 gallon boiler doing 90 minute boils and trying to get 11 gallons into the fermetner. If you, for one reason or another, wanted to do some longer boils, say for a Bopils, a 15 gallon boiler won't cut it. But even for 90 minute boils, the extra room in a 20 gallon kettle is a wonderful luxury.
I have done it both ways and if I had it to do over again, I would have only upgraded equipment for 10 gallon batches once.