Suspension for both front and back necessary?

Normally you replace suspension components when they are worn out or damaged. It is normally apparent if there is a problem because you would have poor handling or noise from the suspension. To check it you would put the vehicle on a lift and inspect it for things like rust, damaged parts, worn or broken bushings, leaking shocks or struts, broken springs or bad mounts. However, there are shops that would tell you to replace the struts or shocks simply due to age and not actually look at them or even do something as simple as a bounce test. That is easy to do yourself, pick a corner of the van, now step on the bumper or put your weight on the car and push down, now let the van come back up and repeat a few times to get it moving, now let go and see if the van stops moving quickly. Less than a small single bounce is good. More than that can indicate that the strut or shock is worn, however it doesn't necessarily mean it has to be replaced immediately. What should be replaced would be if you put your fingers under the strut boot and find a lot of oil, or you can see an obvious leak, then you would replace both of the struts or shocks on that axle.
As for the 9-hour trip, most people would likely drive a couple hours, top up with fuel, stretch their legs and maybe look over the vehicle for anything strange. So, the trip really isn't much different than a normal month of driving. I would probably say to get the oil changed before the trip and have the shop look over the suspension and brakes, don't tell them you are going on a long trip and you are worried, just that you want an idea of any problems to look at in the future.

https://www.2carpros.com/articles/checking-shocks-and-struts

No real way to tell about the struts without seeing them. Normally a bad strut will be oily and grimy on the outside from the oil leaking out. The sway bar is likely the end links. Those tend to wear out over time, usually you will hear it as a rattle or thunk as the suspension moves. Oil leaks depend on what is actually a leak. Many engines develop damp areas of seepage over time that are not really something major but then you can have a leak that drips oil as well. Does the van leave oil spots if parked overnight? If yes, then it still has a leak. Coolant leaks won't affect the A/C, they would be the engine coolant. Like the oil, if it is losing coolant in the reservoir or leaving spots then it needs repair. Otherwise, just keep an eye on them and keep them topped up. Tire leaks - If they were just put on take it back and complain. Sounds like they didn't clean the rim very well. A quick way to test that is to take some soapy water and spray both the inner and outer areas around the rims and on the tread and look for bubbles. Those would be the leaking areas; the alloy rims can corrode and then they leak. You use a wire wheel to knock that off and then apply some bead sealer to take care of them. I might suggest taking it to a different shop and have them inspect it. See what they say. If they give you the same list, ask them to show you the areas they are talking about.

It is usually a judgement call on leaks and such. I look at a vehicle with say 100K on it and I expect it will have some oil seepage from the old gaskets and seals. My general rule is that the less there are the better but if it's weeping and burning say a quart between oil changes, I'm not overly worried. Could it be repaired? Sure. Does it have to be right at this moment, probably not. With suspension parts I look at how the vehicle is being used and overall condition. Then I look at the struts, springs and such. A strut that has been damaged physically I would change, unless it's just a small ding. That is because of the way they work and with a strut being a major suspension part I want them in good shape. Minor seepage isn't a big concern as long as they still function in a bounce test. I've always tried to be fair but thorough and inspect a vehicle that way. So, if the brakes are in good condition and are worn about halfway, that goes on the report, minor seepage at a seal, write it down, worn wiper blades, on the list. Then I go over it with the owner as to which things to prioritize and which I just want noted so that if you come in 6 months from now I can look and see, "Yep that seepage is still the same" or "HMM those right front pads were good last time and now the inner is very worn, might need a brake service if the pins are stuck". However, I don't look at slightly worn brakes and say, "Well it needs new brakes and calipers and these 3 lines, plus an alignment and an oil change with intake cleaning". I do however try to use OE or higher quality parts in most cases. With the current parts quality that is getting harder to do though.

The thing with leaking struts is that as they fail the suspension can get bouncy because they are not doing their job. That can cause tire wear and possible loss of control if the conditions are correct. However, it depends on the leak size. The end links connect the sway bar to the suspension. Not an uncommon failure item. A sort of backwards way to see if you have a bad leak is to wipe off the areas that are damp. Then just them with flour or baby powder. Now if you see a line form where the items meet but it doesn't run or get larger real fast, that is seepage. If however, you dust it and start the engine and a day later there are streaks where fluids have washed away the dusting, that is a true leak.