I am thinking of buying a 2004 toyota sienna is it a good van?

Since it's not a 2010 or 2011, you are safe from over acceleration or under-body tires falling off. But the 2004 did suffer from front driver doors falling off because of cheap manufacturing and defective welds on the door structures. Other common electrical problems in Toyotas are van door sliders breaking, and poor interiors made of plastic which now make squeaking noises inside because of the cheap interior plastics. The paint is bad and chips a lot, and some of them have engine sludge failures. Buy if you can live with those defects, you should buy it. Those defects are very small compared to the new 2011 Siennas which over accelerate, have brake failure, and steering issues which cause the Sienna to be uncontrollable when traveling over 40 mph.