How many more miles do think this car will last

So hard to say with out looking at the car first hand. I can only suggest that you get a mechanic to do a visual inspection and report, if the owner is genuine about the condition of the car, they should not have a problem with this being done. Toyota is a very reliable car, and with it being maintained and any small problems being attended to promptly, normally you would expect to get many more years out of this car, but get a check done first just to make sure that there are no hidden problems.