I am in the process of building a new system using 3 kegs.
I currently have a keggle I use as a BK, and coolers for HLT & MLT.
A lot of folks use coolers for MLT's for the insulation. I'm going back to a keggle MLT because my cooler cracked and I know the keggle will be more durable.
There are MANY threads here showing rigs and how to build your own. Search and you will find tons of info.
Ed