The first thing you need to consider is do you want the other side to know the costs or not?
If the roof replacement comes in at $8,000, do you want the seller to know it isn't $12,000?
If you know that it's only going to cost $8,000 because we gave you a real bid, you might be able to ask for the seller to discount their price by $12,000 knowing they might try to meet you in the middle and therefore they're covering most of the cost. So there are times when it makes sense to not pass the bid onto the Seller.
But having the report shows validity in your request. When the seller sees a real bid, they will take your request more seriously and be more likely to honor it.
Remember the Seller needs to provide a disclosure statement and now that you have made them aware of all of the problems in the house, they will need to disclose all of them to their next potential buyer if you're willing to walk away.
A general rule of thumb for negotiating is "whoever wants it more loses" which means if you are unwilling to walk away from the home, then you lose negotiating power. Even if you're committed to buying the home, giving the impression that you're willing to walk away helps in negotiating.
If you ask for the seller to for too much, then they may respond from emotion and become unreasonable, it is better to come across to the seller as: