I ran an ISF for 2 years between 2014 - 2016, then moved into a gs450 f sport and from that to a golf r (what a mistake, it went to the dealer 8 times in 5 months for warranty repairs so I couldn't wait to get rid!), the golf was then traded against my GS F. I was actually looking for a late model ISF but nothing came up that I fancied so I took the plunge and went for a GSF, very happy I did, it's a great car! Only minor complaint I have had is squealing brakes which are a lot better now the chaps at Lexus Liverpool have de-glazed my pads (I believe the GS F you have put a deposit on has already had the pads de-glazed a few months ago for the same issue according to what Mat told me. It also had a new rear tyre for a puncture.)
The guys at Lexus Liverpool are great, they have 2 master techs working there who have around 50 years of experience working on Lexus cars and really know their way around F cars (I prefer going to them than Lexus Stockport which is only a couple miles from my house).
If you want any other info give me a shout, or PM me.
Good luck with the purchase.
And as a final word, anyone considering the black Gsf at Milton Keynes, inspect it closely, pretty sure it has had a proper trashing around a track which is why it has stone chips all across the front and has had 4 new tyres plus a set of discs and pads in 7000 miles! @BigRat may have some other info in his F database...