Pop keg as boiler? Price to offer restaurant?

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Well I'm always on a look out for a new boiler for my potstill these days.

On the weekend, I was at a restaurant... they have like 20 empty Pepsi pop kegs out back by the washroom. They look like they'd be great for a boiler as I won't have to burn off the top.
Can someone who knows their pop kegs, fill me in if pop kegs make good boilers? In general, do they have plastic parts, are they stainless? Are they easy to mount a heater in?

Even if they aren't good for a boiler I think I probably want some to dispense homebrew beers out of.

For those who have got kegs from restaurants, how did you approach them with regards to buying them? What did you pay for yours?