First let me say I have no real experience with this besides buying 5 used cars in the past 2 and a half years
Will the van come with some type of warranty? If a warranty isn't included I would try to see if you could budget to pay a little more for some type of warranty. Could you have someone look at it first?
Since it is a rental I would definitely just double check that it has all the standard features a Sienna should have. I am starting to think the 2008 Impala we bought in September 2008 could have been a rental (we didn't ask, they didn't tell) because it does not have side airbags and most come standard with them. I have also seen on another thread here about another Chevy rental (a Cobalt I think) not having side airbags.
I think a newer Sienna should have side air bags but I would just make sure you see the word "airbag" where they should be.
As for mileage wise, we bought it in September 2008 and it was a 2008 Impala with about 17,000 miles on it and we assumed it was about a year old (and it now has almost 50K miles on it
)
I currently have a rental (someone hit me and my car is getting fixed) and I am extra careful driving it. I know I could be in the minority but it drives totally different than my car and has different blind spots so I am being extra careful. And I noticed it also does not have side airbags (it is a 2009 Chrysler 300 and when I did a quick google search it did come up optional).
I think anytime you are buying a used car and there is that uncertainty of if the person who owned it before you partied too hard in it.
When we needed a second car last summer my boyfriend ended up with a 2000 Buick LeSabre and we paid extra (around 500 I believe) for a 6 month major part warranty. We figured if something was broke and they tried to do a cheap fix it would go within those 6 months.
I would probably still consider it but I would be cautious. Hopefully someone else can give you better advice.
ETA: I was typing this for a long time but to add to the PP reply - I am extra careful with the rental because I am also scared of getting a scratch on it!