From all the posts I have read, I would avoid the diesel. For your short journey to work a petrol Lexus would not be too expensive in fuel. Test drive both variants but the petrol one will more than likely be less problematic in the long term.
Others on the forum may disagree. If it is going to be your second car, just for work, then you are going to travel a maximum of 100 miles a week. On that basis I would consider an LS - you would not be disappointed.
Here is an example of what you can get -
http://www.autotrader.co.uk/classified/advert/201511248947745?postcode=CT14%209TE&search-target=usedcars&sort=pricedesc&onesearchad=used%2Cnearlynew%2Cnew&price-from=3500&make=lexus&page=1&price-to=5000&model=ls_430&radius=1500&logcode=p