JNAV,
Keel just refers the the center structure that runs bow to stern on a boat. Power boats have keels too. Sailboats can have a full keel (the part below the hull running the whole length and depth), a partial keel (running part of the length but full depth), a stub keel, like yours (running part of the depth, the rest of the depth, referred to as draft, made up by the "swing keel" or a "daggerboard"), a centerboard (the swing keel makes up all the depth below the hull, or a daggerboard (meaning the retraction of the board is up and down like a stabbing dagger), a fin keel that is very deep but narrow like a dorsal fin. There are other terms thrown around like 'wing keel', 'canting keel', etc. that are either very specialized or interchangeable with another term, I loose track and try to stick to the basic ones I'm sure of.
The main purpose of a keel below the hull is to provide two things: added stability by putting more weight as low as possible and lateral resistance, so a sailboat tracks along her path instead of blowing sideways. The swing keel and daggerboard are often not weighted because their primary job is lateral resistance. Your boat has a stub keel and it sounds like you have a weighted CB (centerboard). There will be ballast in the stub keel, maybe lead, also. MacGregors use water ballast too, I don't know where the ballast tank is, could be in the stub keel but probably just over it. Never heard of anyone filling a CB with water. You should be able to sail your boat with the CB up in light winds. My mariner is self-righting with the CB down, your boat probably is too. It might also be self-righting with the board up, I couldn't say.
I would defer to everyone else on this site as to what type of Mac you have. She's good looking and free, that makes her perfect.
You should make sure she is dried out and look into every nook and cranny you can squeeze into. Note where there are water stains, look for rot in the wood, if you have any of that high maintenance material onboard. You can tap the hull and deck with a plastic screwdriver handle to listen for 'dead' areas that might indicate rotting or wet core material or hollow spots where the fiberglass is separating. Pay attention to high stress areas around chainplates where stay cables pull hard on the hull, the mast tabernacle in your cabin top, the gudgeons on your stern that hold the CB should be solid. It looks like the mast step is supported by that 2x4 with the coat hook screwed to it, in the picture with your wife in the 'V' birth. It sits on top of the centerboard trunk and braces the mast step.
leaks most often occur around the portlights or deadlights (the windows - 'dead' means they don't open), the mast step, anywhere something passes through the 'skin' of the boat, the joint between deck and hull, and the pin of the centerboard.
Look for rust, wear, fraying of rigging. Kinks in cables are "no bueno". Jiggle as your hardware, cleats, winches, cams, blocks, etc. to be sure they are seated in good bedding and solid. Don't want to find out your bow cleat is loose when it comes out and you have to dive for the anchor.
Standing rigging is all the rigging that doesn't normally move: stays, spreaders, turnbuckles, etc. Running rigging is all the rigging that does move: halyard, sheet, downhauls, outhauls, vangs, etc. Inspect their wear as you setup.
Listen, do you hear cracking of tiny dry fibers as you bend the lines or sails? UV kills boats and equipment. You have a boat that has had a lot of UV exposure. There WILL be spider cracks, don't worry about them. They are good indicators of places to look for weak spots but they don't mean there are weak spots. Mostly, just make sure all the points of connection for rigging to boat are solid. You will discover anything else over time and most of it will not be critical.
Your 40lb thrust motor is good to get from trailer to rigging dock and not much else on a 25' sailboat. You probably don't need more if you are on a lake and don't have to fight the wind. A long boat hook as a driving pole in shallow water might help to push your bow or stern around when the wind is pushing you off the trailer. 12 volts should work fine for that motor. Over 55lb thrust and you would need 24v. A solar charger is awesome. I'm looking at putting one on Dragonfly. If your system can handle the draw, I wouldn't upgrade until you needed to replace the battery and only if you felt it was inadequate. The mantra is: adequate is perfect.
I will pass on the advice I get form the mariner group I am a member of; Do what you need to do to get her sailing and go sailing. The rest you can do over time.
Have fun.
-Will (Dragonfly)