Yes they are that reliable.
I would try and test drive both the 400 and the 430 so you can make up your own mind. Personally I would have the 430, a major change over the MKIV LS400.
Worth posting on the US forums to get advise, UK doesn't have that many LS owners and you can get better advise about specifications (as they vary from country to country) and the price to pay etc.
http://us.lexusownersclub.com/forums