You don't say what kind of engine work was done or why the transmission was replaced.
I would be kinda' leery of it but just because I don't know the history or the reason for the heavy repairs.
If the van has been abused something else can break at anytime.If the van had an easy life and something just broke then it might be good for quite awhile.
The owner could have been ripped off by the shop and the tranny was replaced for no real reason.There ARE problems with the transmissions on these vans that can be fixed without rebuilding the whole tranny.
Look here for an idea on a fair price.
http://www.kbb.com
Here's a good place to look for mechanical advice.
http://www.allpar.com/