Disappointing the above post has lost all its spacings. Now it's quite a chunk to read.

It was mainly written about Anglican parish church chests, but such chests were widely used by others also. By the mid-19c they would old-fashioned, but still serviceable and many continued to be used. Being old-fashioned, their price would have declined, so they could have been acquired inexpensively, which might have appealed to your church's Elders.

Wouldn't count on the rest of the churchyard not having any burials. This possibility is probably covered in your sale conditions. Might be wise to talk to your county archaeologist about what to do if you encounter human remains. It is not an uncommon situation for civil engineers and builders.

Burning is about the last method that would be used to open such a chest. If it has to be forced, drilling is usual.