Using coolant additive

I went to look at a 2001 Elegance Octavia TDI today (being sold privately). A nice car, good condition drove well etc.

One thing concerned me a bit though - On openening the bonnet I noticed that the coolant was not the normal G12 colour.

It was a dark blue / green / brown colour, which the owner told me was due to the skoda dealer putting "greased lightning" coolant additive in (which he showed me a bottle of) at the last service. Apparently the dealer recommends this on cars of that age.

according to the bottle, it:

"lowers radiator temperature, stops leaks and radiator corrosion, extends the water-pump life, and is a coolant and anti-freeze booster."

and contains PTFE.

This surprised me as I thought the cooling system should be fine on just G12, and is causing me some concern, either that it's being used to stop a leak, or that the additive itself might not be good for the cooling system. Any opinions on this?

Also, what do you think a fair price is for a 2001 Elegance tdi hatch with 90k miles?

Thanks

Jon