Express vans, 1st generation using the old small-blocks, in 1/2 tons you will find 4.3 V-6 with options to 305 and 350 V-8. I think all will have 4 speed OD automatics, but in different sizesaccording to engines. 3/4 and 1-ton will be at least 305, most are optioned to the 350, while big block was available but rare. All of these were Vortec branded, 4300, 5000, 5700, 7400, 8100 and all have 4L80e transmission. I would want the transmission upgrade if thinking about towing anything. even a small boat.
2nd Gen, 2003 forward, got the new small block V-8, 4.8, 5.3, 6.0. 1500 got V-6 standard, 5.3 optional. 2500/3500 got 4.8 standard, 6.0 optional. The 6-speed auto was introduced in 2010, with the 6.0, on 2500 and 3500 only.
V-8 diesels were available, 2500/3500 only.
Shopping in 2011-2012, I found most used 2500 Express vans were equipped with the 4.8, while 6.0 was easier to find as a 3500. Used, there was not much difference in price, 2500 vs 3500. There is more of a price premium for the long wheelbase van, which you will find more often as the one-ton.