Done it many times, as said, the enclosures are MEGA expensive, very heavy, and mounting anything in the cover requires that the enclosure mfr know in advance so they can drill / tap holes. Then if you need anything that requires viewing,you have VERY expensive Ex. Proof windows to have installed. Lastly, heat produced by components inside can be a huge issue. That's why a lot of people say it's usually cheaper / easier to put it in a non-classified area. I can't even count how many times I have gone through the exercise and the end user hears the price, so they come to that conclusion anyway.
The last time someone asked me for that, I priced up a regular control panel, then just added $50k to do it in Ex. Proof. They decided to move it, so I didn't duplicate my efforts.
Purged systems are not cheap either and require specific listing capabilities.