If it was easy for an amateur to open, it wouldn't be much of a safe, would it?
Get a quote from a local safeman. You may be able to negotiate the price down by letting him fit you into his schedule rather than vice versa. If the price is still too high, you may be able to negotiate a "you can take the safe, we keep whatever was inside of it" deal, which will satisfy your curiosity at minimum cost.
OP says that it's a key lock. He may have more luck at the local deeds office tracing the previous owners or maybe local estate agents could point him in the right direction. On the other hand the previous owners, if he finds them, will probably laugh and say
-- What key?. If you just want to get in regardless of damage then one of the modern diamond loaded blades will probably slice straight through the steel and concrete back. Talk to your local concrete cutting people.