M105 is more of a rubbing compound. It is easy to get out of control and in many cases it will dull the finish which scares a lot of people. Since it cuts pretty fast, you really have to pay attention to seams/high spots/transitions and if using power tools... a few seconds of space cadeting it and it's a lot of misery. If you have a lot of swirls/heat/orange peel in the paint you can quickly get them out with M105.
Then you need to finish it with a polishing compound. M205 *appears* to have less oils/crap to keep the cut material suspended than the Ultimate Polish. M205 is faster but you have to pay more attention and you have to start clean more often.
The Meguiars Ultimate Polish is good enough for everything, but I like Old No 7 better. Old No 7 is more forgiving (and doesn't dry out as fast).
The only time I really use M105 (or any other rubbing compound) is if I block sanded first.
I'm mostly looking for "it looks nicer than fresh off the assembly line" and not show car quality. There are way better and way more labor intensive (and expensive) ways to get better results. Few people really notice so why bother?
Rejex is just a polymer sealer. It wipes on and wipes off. You can use a machine or do it by hand. I do it by hand. It doesn't fully cure until about 12 hours after you remove the haze which seems strange to most people. If you've used Meguiars NXT 2.0, it is actually easier than that. It lasts nice for about 3 months and at 6 months you can tell it's still there.
A $6 bottle of Old No 7 or $11 bottle of Meguiars Ultimate Polish will do a couple cars. A bottle of Rejex at around $20 will do about 4 cars. I usually cheat and use Scott white painter's paper towels (they're soft like toilet paper) for polishing and sealer application. Use microfiber towels to remove the sealer.