I also think the facelift model has Bluetooth and on some cars you may find adaptive radar cruise control. I also believe they added cornering headlights. The 6 speed box increased the torque so the 0-60 time went from 6.7secs to 6.3secs. Fuel economy was also slightly improved.
Pre facelift cars are obviously going to be cheaper than facelifted ones. I don't think either disappoints though, to me condition and service history will always win out. In other words if there was a good facelift car and a REALLY good pre facelift, I'd take the pre facelift.
Also, when checking the bonnet struts, check the boot ones too. I had to replace mine but they only cost £25 on eBay and an easy 5 minute DIY job to fit. As others have said go over the car carefully and don't rush, check all the gadgets and electrics (there's lots on these cars!!!). I too had many problems with my Rover 75 which is a shame because they are nice cars. You really can't beat the quality, fit and finish and refinement of the Lexus LS though in my opinion. I also love the attention to detail these cars have such as lights under the mirrors that automatically illuminate the pavement as you approach the car, illuminated interior door handles, even the storage box under the armrest is illuminated. There is a tiny 3rd sun visor above the interior mirror, they have soft closing doors and much more. One thing I would suggest is look at a couple because even ones that are a bit rough can still easily seduce you, they are that good. The engines and gearboxes on these things are pretty much bombproof and will run forever with regular servicing. These cars are over engineered but there are some weaknesses as pointed out in posts above, I don't think the problems with these cars is that anything is unreliable as such it's just if things like the air suspension, steering column motors etc do go wrong they are very expensive to fix. I've decided to enjoy my car as it is and not worry about something that may never go wrong, I'll cross that bridge if and when I may come to it. Good luck with the search, just don't rush and remember to check everything.