For engines and transmissions you have a few choices.
With engines you can get factory new for a couple hundred more than a rebuild. The factory engine usually comes with a 1 to 5 year warranty while the rebuild will have a 30 day to 1 year warranty. The rebuilt engine requires you to send your engine in as a core. You can "freshen" your engine by having the block bored and having new pistons to fit the larger bore. You will need a new water and oil pump and a new cam and lifters. The nice thing about freshening an engine is that you can fit it to your use. raise the compression ratio to around 9:1 for a bit of added power AND better mileage. A high volume oil pump will supply oil at the same pressure but will make up for bearing wear as the engine ages. A larger oil pan will keep the oil cooler and make everything last longer. The new cam can be a roller cam - better mileage and power suited to how you drive. The down side is that unless you do the assembly the refreshed engine is likely to cost slightly more than a factory new engine.
You might be able to get a factory new automatic transmission or a rebuilt transmission but you have to be careful about rebuilds because most "rebuilt" transmissions are not rebuilt. Most often they are resealed and have new friction parts but the bushings are not replaced. I used to rebuild hot rod transmissions and I replaced all the bushings and thrust washers as well as installing new steel and friction parts. I would add to the number of clutches, increase the pressure slightly, increase lubrication and cooling capacity on every transmission I built. Since my accident and resulting disability I sold the business to my two employees.
Without knowing what year your van is it is impossible to provide you with any accurate pricing but for factory new engines you will be looking at $2500 to $4000 for an engine depending on availability and size. A C4, C5 or C6 auto is only available from the rebuild market and the best place I can recommend is
Broader Performance in Texas if he is still in business. That is where I would go for a transmission. His pricing is more than fair and if you call before noon he will be able to give you exactly what you need. Do plan on getting the largest transmission cooler available to safe guard your transmission. An optional deep sump pan is also good for adding to the longevity.
If you have a manual transmission then
David Kee is the man to go to.
Both of these transmission businesses will sell and ship to anywhere in the states and some export locations. Their prices are more than fair and quality is first rate. I can't recommend any engine rebuilders because I have always done my own engine work. I think I would check out local NAPA stores as some have in-house machine shops and their prices are competitive and quality has always been good. Check how happy people have been with the work and what their warranties are.