For 15 gallon batches you will need a bigger pot. For pot size i would suggest about 50% more space than your batch size.
You will want some way to heat the wort (propane burner or electric)
You will need to have a ball valve on your setup, you need to move your wort with tubing and lifting 50+ gallons of hot wort is a bad idea.
Chilling will be a bit of a challenge, a huge immersion chiller would work, otherwise plate or counterflow chillers are an option, but you are likely going to want a pump with one of those, as I highly recommend a whirlpool before chilling to help drop out the trub. Actually a whirlpool for a batch that big is probably a good idea anyway, and if you go for a IC it will help reduce chilling times.
For fermentation you can use 3 5 gallon fermentors, otherwise there are conicals, or even 20 gallon+ buckets (used in large scale wine making)
As for recipe there are some differences in hop utilization (less hops needed for same IBU) are a larger scale, not sure if 15 gallons is enough to for that to start being noticeable though.
Don't take any more faith in anything I say than you would anyone else on the internet. If you listen to what I say, then hurt your self or break something it is your own fault, I am just expressing my opinion or experience.
THINK for your self!!