Step one should be the use of a good oil flushing chemical/detergent. (I used AC-21 oil flush and the amount of crap that came out with the old oil was both very impressive and somewhat disappointing). Otherwise, you're likely to overload your fresh oil with crap that can easily be removed prior...
I have two vehicles. One with a belt the other with a chain. I have replaced both the belt and the chain. The belt cost about $100 (including tensioner) and took about 30 minutes to change. The chain and tensioner and gears (which all wear together) cost about $500 and took about 8 hours to...
My Toyota charges at 14.4v at all times. (I have a dash mounted digital volt meter). It has done this for over 13 years. In this time it has only needed 3 batteries. So, IMO, there is no problem with op's charging voltage.
I use two sets to lift my car up a couple of inches while keeping it level to do an auto transmission final ATF level check (thanks Toyota, you muppets). However, for anything else maintenance related, they're essentially useless.
Those ramps will barely provide enough height to swing a wrench and place an oil drain pan under the vehicle. I'd recommend ramps with a 6" lift for oil changes.
Only if the new plug is over tightened. When you install the new plug either use a torque wrench or, if you use a regular ratchet, just grip it up near the head rather than using the full leverage that holding near the end of the handle would provide.