I tried to buy a rear facing seat.
Manufacturer of seat sold a fixing kit for my car make, model, year range of manufacture.
The retailer said it Should fit my car.
The retailer spent 45 minutes or so, trying to get it to fit my car.
It did not fit.
ISOFix - rubbish in my view, it isn't totally universal, sometimes thing just don't fit. Fault of the car manufacturer, fault of the product manufacturer, just some weird reason to do with the way the seat cushion is fitted, who knows... end of the day, the seat would not fit.
So even if you try to fit an expensive seat which attaches to the chassis of the car, there is no guarantee that it will fit. Thus never buy online, always get it fitted by the retailer - so you don't get billed until it's fitted.