Hi although I have never seen the vehicle common sense dictates the question do I throw more money at it, if the garage wants to pay the cost by all means, as Clifford suggested fit a reconditioned unit as this won't have the problems you have now, with regard to the car being clapped out it must have looked like a good proposition for you to buy otherwise you would have walked away, the brown emulsion in the oil is a classic example of fuel ingression, if you want to replace big end shells you would have to replace the piston assemblies as well ergo a recon unit would be better its known as a short engine which has had the crank journals reground and oversize shells fitted, new oil pump, pistons and rings, personally as Gareth suggested I would see if I could get an engineers report, get my money back, you may suffer a small financial loss but its better than losing the lot and if you source another vehicle either buy a couple off years later model or buy a diesel as its impossible to buy one with that type of problem as they rely on compression detonation and if the rings are gone will run like a bag of spanners.
Steve.