your not after much are you?...

its hardly the type of thing you have lying about in a garage..

a 60 inch possibly marble hearth (what else is there apart from granite and/or limestone?).. mdf would be no good if its a gas fire thats in place or going to be put in - but if its an electric fire then you fine..although an mdf hearth is what it is..cheap and nasty looking...and its mdf!!!
and what type of material is the surround? and what type of back panel do you have if any?? - bearing in mind you may want the back panel and your new hearth to match?? and to look ok with your newsurround..
good luck to you..its a minefield out there..