Air Conditioner Problem

You are misinformed about something much more important. The "Oil" warning light refers to oil pressure, not level. Oil pressure can be low if the oil level is too low, or the sending unit can be defective, but at the mileage you listed it is much more likely the bearings in the engine are severely worn. That is a common reason to sell a vehicle and dump that expensive repair on an unsuspecting buyer.

If the battery is weak or has a shorted cell, it will crank the engine slowly. Once the engine is running the battery will not cause stalling. I had a hard time understanding that comment.

For the AC system, you need to provide some test results to know where to start. Everything could be dead due to a fuse / wiring / electrical problem. It could be disabled by the low-pressure cutout switch that prevents the compressor from running when the system is low on refrigerant. Those do not have anything to do with the condition of other systems. If the AC system was simply low on charge, you can be sure the seller would have had it charged so it would work and he could sell the van for more money. More likely there is a leak that is very expensive to repair.

It sounds like you are looking to buy a project vehicle that is going to need a lot of time and money. If you will need to pay a mechanic to diagnose and repair the problems, be sure to factor that into the price you offer for the van. I think you are going to find you will have two or three times what it is worth in it when you are done.