This would be your auction bought finance snatchback with no oil in it? Tell us how much you paid and we'll tell you wether it's worth bothering or not!!
Firtstly,are you certain it's the big ends?- they make a very distinctive heavy metallic knocking sound and in your original post you said it was a rattly/clattery sound. If it is the ends knocking, then it won't be cheap- it's a full strip of the engine, new ends (which cost pennies to buy...but you need to get to them..) the crank will need a regrind, and you'll need a new oil pump-at minimum. There'll be swarf in the sump,it'll have got into the rings'n'bores.....I could go on but you get the drift...
If the motor's shot, your best option is to source a good secondhand engine and just swap it over.