Hi GlennK, I had trouble finding a pot that was big enough too, as most the pots big enough can't be used on a glass top stove. I ended up having to order what I needed from Amazon. So I have a 21 qt pot that I put into a 36 qt pot.
To heat I used my stove. The warmer burner is perfect for maintaining temp. Medium for the 80's and high for the 90's. I use the sink for the initial fill at the required temp and the warmer to maintain it. When it's time to increase temp gradually over a period of time, I slide the pot over a real burner and turned to low or medium as needed. That part takes some practice as every stove heats different. I turn the stove off after the water is heated 3 or 4 degrees past target temp and let the milk catch up. The water/milk should equalize at target temp and then you can slide it back to the warmer for maintenance.
Once you find the pots you want (people who sell brewing supplies have good ones too), you should fill them with water and find out how you stove heats, so you know in advance of making cheese. It takes a little practice to get perfect. If you have a glass top, realize that the weight may hurt your stove... I'm pushing limit!
I really only use the sink to bring my milk up to initial temp. Then I put it in the big pot on the stove that is at the correct temp and sitting on the warmer.
As dthelmers has noted, there are many ways to heat your milk. So the trick is to find a method that works for you!