Hey, welcome to the club.
1. At the speeds you're talking (and about any other) stock condition is probably the best mileage you'll get unless you get into high altitudes. 4500+ feet usually starts to affect jetting. From there you can carry extra jets or get into those Intellijet/Dial-a-jet things. You will be a little displeased with your front tire when in gets slippery, so a Shinko Golden Boy is a nice upgrade that's similar to stock. The stock rear is fine for me, but other folks like a lot of rubber, which may or may not drop your mileage and rub some things raw.
2. You can carry pretty much every tool you'll need for most repairs with you, and there is a thread that I'd imagine someone will know that tells you the common ones. You'll want patches/tubes, a couple tire levers, your wrenches for sure. An extra master link and a few links. Bailing wire is nice, and given it's an '87, I would consider trying to find another CDI. One thing is a few screws on this bike (the brass ones on the carb for instance) use JIS screwdrivers, but look like phillips. You don't want to make that mistake in a pinch and strip them out. As well the screws holding the float bowl should be replaced with allen heads honestly. You can ebay a stainless allen head bolt set that covers most of the motor as well. Remember the JIS screwdrivers though.
3. A lot of us will tell you to just use a bicycle pump and patience. The others both have some drawbacks. Electricity is obviously hard to come by sometimes and the CO2 cartridges sometimes leak when you put them on the valve, or you find you haven't carried enough for the large rear tube.
4. I'm not sure if the '87 had e-start or not, but I do know its electrics are a bit unique, so compare loads and draw on paper before you go plugging stuff up maybe. A nice solar panel I'd really recommend is the SolarJoos though. It's quite literally bulletproof and substantially cheaper than a lot of touring solar setups last I checked.
Anything you haven't thought of has been covered by Mr. Gizmow. Read his posts and unique fixes for common problems. Obviously he's not the only one. These folks are resourceful. Enjoy.