Oil viscosity depends on how much you'll be using it in really cold weather. For weather in the teens (farenheit) use a 5w 30 diesel rated oil for easier starting and for warmer weather most recommend 15w-40 diesel rated oil. (15 qts with filter) Many powerstrokes have a plug-in engine heater that helps a lot in really frigid weather. The oil filter is quite heavy when filled with oil so many powerstroke owners punch a hole in the bottom of the filter with an ice pick to drain the filter before removing it. Just make sure to break the filter loose first in case it's stuck tight. Use a diesel rated oil which has anti-foaming additives or your injectors won't work properly.
Your engine started out with the old-style green coolant and you shouldn't mix in the newer style blue/orange/yellow coolants without doing a comprehensive flush of the old green stuff. If you refill the system with green coolant you'll also need to add about 64 ozs. of supplemental coolant additive (SCA) to guard against cavitation which can make little tiny holes in your combustion chambers. Once the coolant has been refreshed you should check the SCA levels about every 15000 miles. Test strips are available most anywhere that sells the SCA additive. A full flush takes 4 gallons of coolant concentrate (not the pre-mixed stuff) and 64 ozs. of SCA. If you have hard water in your area refill with distilled water to avoid mineral build up in the radiator.
Brakes use Dot 3 or Dot 4 brake fluid, or whatever the equivalent is on your side of the pond. Brake fluid picks up water over time so your rig is almost certainly ready for a full fluid flush. The rubber brake lines that connect to the calipers have a habit of getting clogged with junk and a flush will show which, if any, are clogged.
Your owner's manual says not to use Mercon V in the transmission, but Mercon V was later re-formulated so you can use either Mercon or Mercon V tranny fluid. (18 qts capacity) You may want to drop the trans pan and look for metal chips in the pan. If your pan doesn't have a drain plug you'll have no choice. The pan holds about 8 qts. of fluid and gets pretty heavy. Mine has a magnet inside the pan to grab any wayward metal junk. Check the existing fluid for a burned smell and/or a really dark appearance. Either one may be a sign of trouble in the trans.
Mercifully, replacing the alternator on your engine isn't too bad a job. I generally consider working on van engines right up there with medieval torture, but replacing alternators and turbocharger work is fairly easy on these engines. Many E van based buses came with high output alternators (200+ amps) to keep up with the big-ass air conditioners and wheelchair lifts. Mine had an aftermarket 230 amp unit with non-standard connectors so I had to do some re-wiring when I replaced mine with a "stock" 130 amp alt. Now would be a good time for a new serpentine belt if the existing one looks the least bit worn. I kept my original belt as a spare.
Speaking of turbochargers, some of the early powerstrokes had a problem with the bolts that fasten the hot side of the turbo to the bearing housing loosening up over time. Ford has replacement bolts that allegedly fix the problem (Ford part number 1C3Z-9G486-AA) Mine were totally backed out. Basically, the new bolts have their threads buggered to lock them in place. From what I read, that area gets too hot to use locktite to lock them down.
The fuel filter bowl on top of the engine has a water drain on the side of the bowl and most E vans also have a cable near the oil dipstick that operates the drain. You pull the cable and fuel and water splatters all over the left side of the engine and onto the driveway. Not too neat but effective, particularly in someone else's driveway.
My fuel bowl had all kinds of junk in it when I changed the fuel filter. So did the filters in the fuel pickup in the gas tank.
That's about all I can think of right now but I'll post up anything else that comes to mind.