There is no way to predict the future. Even with excellent care - "stuff happens" - like a tranny failing 1K out of warranty (happened to me).
Your best bet is to know how the car was maintained and driven in the past. Are there service records? Was it driven gently on perfect roads or driven hard on bad roads? Stop & go driving or easy rural highways at 55? Accidents? Was it used for towing (then don't consider it)?
The best thing to do is to take it to your mechanic before committing to purchase and have it fully checked out, with a written report on what it needs now & what it will need soon and the cost estimates. With that in hand, you can begin negotiations, using the Edmunds custom appraiser tool to determine its TMV.
Toyota makes a nice van - but you must do research on the engine to see if it was subject to sludging - check the Sienna board here at Edmunds and also the owner's ratings. Common problems to minivans are power doors and sunroofs. I'd prefer one without those features. Minivans can be tougher to buy than passenger cars, IMO, because they seem to get more severe service as family vehicles than say, a used passenger car that has carried only its driver for 98% of the time and then mostly on commutes to work.