Good morning.
Sorry for my delayed response to this but we've had a few days away in our trusty steed clocking up some miles around beautiful Devon and Dorset and only got home late last night, unfortunately hitting a substantial pothole on the A34 on the way back that i need to check hasn't caused any wheel, tyre or dudpension damage
I understand and commend your thinking regarding trying to maintain the quality of keeping the car original but when it comes to tyres I'm afraid manufactures don't always fit what necessarily is the best but often what they can get the best deal on from a certain manufacturer.
The original tyres on the 16" 6 spoke SE wheels were indeed the Pirelli P6000 Powergy in 185/50R16 81V variety but they were pretty poor and suffered premature crazing and cracking of the rubber compound, evident in the sidewalls and between the treads but worse still suffered stepping where the tread wears evenly which causes amongst other things excessive noise and an uncomfortable ride.
That said I have also had original 16" 5 spoke SE wheels fitted with Dunlop SP Sport 9000 tyres from new in the same size which possibly supports my theory regarding fitting what they can get the best deal on at the time but obviously also could be down to availability or that they realised some tyres weren't too clever and changed to another brand.
As others have said tyres have come a long way in recent years and even the budget tyres now are generally much better than they were and in fact most use the mould patterns from older premium brand tyres so aren't anywhere near as bad or plain dangerous as they once were, not that I'm recommending using them at all btw.
Unfortunately because the factory 185/50 16 size isn't mass produced as they are only used by a few models of cars and therefore produced in very low volumes they tend to be expensive so best bet is to do your homework and check reviews and the tyre labels on a site such as Camskill.co.uk who I use and post options that you're contemplating buying on here before you do
I hope that helps a little