If it's free and you have the storage space, by all means take it.
There is a variety of engines you can fit into that bay (289, 302, 351, etc...). And they are very plentiful, with literally thousands of options as far as parts as well. You'll never have any issues finding parts for early Mustangs.
Even if you need to sell it later on, know that someone will want it.
a brand new shell is about $16000, but that is brand new and ford still makes them. however used bodies can cost anywhere from $500 - $4000. it depend on what kind of motor you drop it in. can cost from $1000 to $10000.
honestly if you do not know any good mechanic or do not have good mechanical/ do it yourself ability I would suggest you buy one that already has a motor and in running condition. it will save you a lot of time and trouble.
you also have to consider the transmission/ wheel tire, steering column, window glass, paint?, exhause... just dopping a motor in does not mean anything if you can't drive it.
if you provide us with your intended purpose of this car's usage it would help..a lot.