It would be bad if one of the conrods has snapped. It's not so much the expense of buying a new one, more figuring out why it snapped in the first place.
Generally a con rod would snap when the piston siezes in the cylinder, in which case not just new con rods, but also new cylinders and pistons would be necessary for a repair. If that were the case you'd be much better picking up a second hand engine. Not the end of the world - I've not seen an engine sell for more than £50.
A very real possibility is the previous owner doesn't know what he's talking about. If the engine turns on the starter motor it is unlikely to have seized! Have you got a manual for the car to check the ignition timing and points gap? (Haynes are pretty good).