99 Sable , loss of oil pressure

[PHF]

I live in Australia and have a friend with nil mechanical knowledge in the US to whom I am trying to give advice about an oil pressure problem in their 99 Sable . as I don't know these vehicles personally I need some help . the car has about 70,000 miles on it and registers nil oil pressure . the car had not been using excessive amounts of oil until suddenly this nil oil pressure fault occured . the engine is making no unusual noises [ a mechanic verified this ] . two mechanics have examined the car , one says it needs a replacement engine the other says quite possibly it is an oil pump . the estimates on the job are approx $5000 for replacement engine , $1800-2000 for a new oil pump . my questions are ,
1) I have read here that the Duratec engines can experience internal oil leaks , what are the nature of these leaks and repair method ?
2) is there any additive that can be added to the oil to seal up these leaks ?
3) what is involved in replacing the oil pump ? I imagine the pump is situated in the sump necessitating the removal of the oil pan . due to being a front wheel drive vehicle I can see this may not be a straight forward job .
4)an estimate of cost of parts and labour ($80 an hour ) to replace the oil pump [ $1800-$2000 quote ] ?
5) an estimate of replacement with new engine [ approx $5000 quote ] ?
thank you ,
PHF