Help needed - Coolant in oil

Hi, I have been the owner of a F430 for the past 4 years and it’s been amazing and super reliable. As a result, I decided to add a 550 to the stable a year ago.
The car had been with the same owner (older gentleman) for the previous 7 years and had not been driven much at all during his ownership.
It received a major service upon the point of sale including upgrading of the cooling hose.

Fast forward a year, and 3k miles of trouble free ownership, I have just taken my 550 in for its annual service at my very trusted Indy who has been looking after my F430.

I have just been told today that some oil has been found into the coolant tank. They also found some traces of StopLeak additive (OMG!!) in the system too. They flushed the system, filled it with new cooling fluid and ran the car to temperature and found that some oil made its way back into the coolant tank immediately.
They have also sent an oil sample to a lab (something they offer with all routine service) and it confirmed a high level of sodium (coolant) in my oil but no metallic wear.

The workshop is telling me that they need to drop the engine and open the block to investigate whether the problem is only a matter of changing the head gaskets and clean everything to remove StopLeak residue, or whether the head unit/engine block have been damaged and need to be rebuild (nightmare scenario).

Although I do trust that he has my best interest at heart (he’s also fully booked and told me he cannot do the work until Jan if I decided to go for it), the 30h of labor to drop the engine just to investigate is a bit scary.

The concern is also the use of StopLeak additive. Why is it used and what consequence could this have on the engine?

I am very puzzled as to why the selling dealer who has been looking after that car on behalf of the owner for the previous 3 years and performed a major service (inc fluids) and upgrading the cooling hose would have not mentioned anything as the coolant was contaminated and traces of StopLeak were clearly visible.

Can you guys think of anything I should ask my Indy prior to him starting the work? What recourse do I have with the seller given the transaction happened just a year ago?

 

Correct - it’s the 550, not the 430. Sorry for the confusion.
Re: exchangers, I don’t believe the 550 has any

 

Looks like it. I really hope the head and block are unharmed. Upon disassembly the head will be measured and pressure tested to make sure its perfectly fine

 

What does leak down the motor mean? I am not familiar with that process?