If you only do short journeys, you may be disappointed by the fuel consumption of the RX400h. This is on the basis that the car will not run on battery power only until the petrol engine has reached its normal running temperature. The time it will take to do this will vary considerably depending on the outside temperature and the demands you place on the car (air con, heated seats, heated rear window, lights etc).
The car really comes into its own in town and on A/B roads once warmed up.
As an indication, my wife tends to use the car on a day to day basis, but only for short trips (less than 5 miles). She achieves betweeen 25 and 26.5 mpg. On a longer run, either of us will get between 27-29 mpg. You will find on this forum that other drivers achieve better figures, but I must say that we do not drive the car to beat records.
Given this, you may decide that the hybrid is not for you after all, but you also ought to take road tax, insurance costs and resale value into consideration. This may tip the balance back into the hybrid's favour,.
With regards to the battery issues, you will find that they tend to affect cars that have an old(er) battery, and/or cars that have left standing unused for a while. Depending on how the car you want to buy is, you could insist on having the battery replaced.