There really is not much to it. These are commercial machines so they are made to be hammered on all day. No special tools are needed. You will need a soldering iron and multimeter at some point but you can get those as you get more comfortable working on machines. Nothing special needed to clean pins either. Novus1 and microfiber cloths works well cleaning playfield and plastics. Just avoid anything water based when cleaning playfield. Wax is optional. Older games that do not have a clear coat on the playfield need it more to protect. Modern games (90's on up) don't really need it imo. Just keep an eye on the balls and replace them when they start to get dinged up.

"Shopped" or "serviced" can mean different things. It should mean the game was gone through, cleaned, and everything fixed. That's not always the case so it really doesn't mean anything. It might mean someone just wiped the playfield with a rag or one burnt bulb replaced. Other acronyms/terms are used to describe condition but they really don't mean anything. Condition is the important thing.

No need to overthink all this stuff. Play as many games as you can and figure out what kind of game you want to get and what your budget is. If you have the funds, I brand new pin should be something you can just set up and enjoy for a long time. For a beginner, I recommend starting with cheaper games so you can build up a collection of 3-4 pins faster and learn as you go.

For older games, ignore most of what seller says. They could claim it's mint or that they restored the game but condition tells all. Condition determines price more than anything. For a beginner, I would focus on finding a game that is working 100% and looks like it was maintained well. If pin is filthy dirty, broken plastics, dry rotted rubber, and some things are not working then walk away. Seller will of course tell you everything is an easy fix and it was working fine yesterday. Try meeting some people in your area and see if they have any pins for sell.