Battery

Lots of things. You can use plain grease, dielectric grease, vaseline, special terminal sprays, felt pads, etc. Those will all help, but corrosion will always occur - just a function of system. Te trick is limiting the amount of corrosion that builds up - quickly cleaning off any signs of corrosion before it gets too bad. Should be put on both the battery posts and the terminals.

When running - alternator should be putting out somewhere between 13ish to mid 14 volts. Exact expected range will vary from model to model - most Toyota's that I've worked with, unloaded, they can run from 13.2V to 14.8V.

Don't read too much into why Toyota limits them to 14.2V - there is a limit, but that is on the the voltage regulator side. Some run a little more than 14.2V, some run a little less. It makes sure that the alternator is not putting out too much or too little juice - otherwise, the alternator would be putting out maximum power all the time. This limit is dependent on a number of things - usually to safeguard the battery from overcharging.

You can still AVO meters. Fluke is overkill, unless you like to have decent equipment, or use one for work related projects. Old RadioShack analog meters were surprisingly good - too bad you can't find them any more + many people don't know/don't remember how to use those types. As for functions - a basic multimeter will get you 99% of what you need. High impedance would be nice, most modern digital meters, even the cheap ones have a decently high impedance. Most of what DIYers use these for is voltage and resistance - everything else is just "nice to have".

Its more of a question of what you can afford and what you need to use it on. If it is jut general all-purpose type of work around the house and automobiles - then almost any, inexpensive DMM will be fine - $20 to $30 is probably just right, even Harbor Freight $10 meter probably would work just fine. If you are looking for a meter with very high impedance, very high resolution because you are looking for very low voltage fluctuations - then maybe a $6000 Tektronix meter is the way to go.

I'm pretty sure we talked about this at length before - might have been through PM. Anyways:

High impedance refers to the circuit built into these meters. Higher impedance generally allows the meter to be placed across a circuit without adversely affecting the measurement or circuit under test. Almost all digital DMM are atleast 1MOhm impedance, but would be a good idea to check anyways. These are safest around sensitive electronics and control circuits, but can pickup so called "ghost voltages" or phantom voltages - not a big deal for most people, but it is something that cause you headaches if you use higher precision meters. Some meters are low impedance, like the old analog one - great for power circuits or if you want to load a circuit down.

I have a wide range of meters, different one for different occasions. Since I use many for work, I can justify the cost. My usually go to meter is a Fluke 87 - considered an industry favorite. But I'd be hard pressed to recommend someone dropping $400-$500 on one of those meter to work on the car / around the house. Even used ones can go for a couple of hundred bucks - that buys a lot of parts / gas for the car.

I have a $40 from HomeDepot that I like to use - works well enough to get me the information I need. Really compact, just eats batteries quickly. Actually has a really nice feature Fluke doesn't have - can trace electrical wiring through a wall and flash if the line is hot.

Should be indicated on the packaging / documentation with the meter or sometimes printed on the meter itself.