A few clarifications:
2. Add ASW for escorts to the list, otherwise it's just AA and radar for capital ships, and those and torpedoes for subs.
3. Upgrading INF to MOT will provide some mobile practical, but less than if you built the MOT from scratch. If you have a highly experienced INF division, it may be worth upgrading, otherwise I'd build a fresh division.
4. Division design is a complex issue, depending heavily on what country you're playing and who you're fighting. Normally you'll want at least 2 INF brigades in an INF division for the higher toughness and defensiveness. For most of my games, I run 3xINF+ART, with a few divisions of 2xINF+ART+AT to deal with enemy armor. If you're playing Japan and fighting against China, for example, you don't need the AT. If you're playing Germany facing the Soviets, or vice versa, it's essential, although not necessarily in every division. Be mindful that AT has very weak Soft Attack, and is the worst thing besides MPs that you can put in a division that's facing an infantry division. Since MOST of the divisions you'll fight will be INF, putting AT in every last division costs you a lot of firepower and a division slot that might have been better used in something like half of your divisions. ART will offer more sheer firepower than another INF brigade, but provides less Toughness and Defensiveness, and costs more IC but less Manpower, so your choice will depend on what you can afford and what you're facing.
4b. Armored divisions only need ONE armor division of some kind to provide the division's Armor rating (which can be defeated by the opponent's Piercing stat), that provides a large set of penalties for any opposing divisions unable to pierce it. Armor is expensive to build, and expensive to supply, so you don't want to put 2 in a division if one will do the job. If you build the division with Light Armor, its main advantage is speed, so you want fast brigades to go with it for exploitation purposes: MOT and Armored Cars. If you use ARM, then it's got better Armor and Piercing stats, and is better suited to attacking, not outmaneuvering, and you can saddle it with SPART or TD without losing much speed. HARM is a specialty unit, slow and VERY expensive to build and supply, but VERY powerful, so you want to maximize its primary strength of low softness by accompanying it with MEC and SPART, which have decent hardness of their own.
ENG, AC, MTN, and GAR have their places as specialty units, but aren't something you want in quantity. I generally try to put one ENG brigade in one division per Corps, because they're clearly superior against fortifications, river crossings, and dense woods, but as pathetically awful in the open plains as AT is against infantry. They're something that you really want to have one where and when it's needed, but more than a handful or so in your army is a waste. AC provides a speed bonus to its entire division in several types of terrain, so a fast exploitation division (Light Armor) can benefit greatly, and it helps lower the division's overall softness. Basically, it's a cheap filler brigade for a LARM division, especially before SPART is available. I also use them with MOT or CAV to park on airfields (AC is a "support" brigade, like ART, AT, HQs, and SHARM, and can't fight without being combined with a "line" brigade), to insure a supply of fuel BEFORE I rebase aircraft there, and to chase down occasional Partisans. MTN and MAR are great for their respective Mountain and Amphibious Assault bonuses, and have slightly higher ORG than normal INF, but their higher cost prohibits fielding them in large numbers. They also fare badly against Armor. GAR are best parked in ports or occupied/annexed cities that you can't afford to lose, and reduce Revolt Risk slightly in the surrounding provinces. They cost slightly less than INF, use 1/3 the officers, eat considerably less supply, and hold the province defensively almost as well as INF.....just don't ask them to go on the offensive or go anywhere quickly. MIL moves a bit faster, fights offensively somewhat better (still considerably worse than INF), defends worse than GAR, and provides no suppression of revolts, but uses 1/10th the officers of INF, is somewhat cheaper, and is frugal with supplies: best used in areas of really pathetic Infrastructure where regular INF will suffer shortages, or in huge waves that overwhelm by sheer numbers to offset their limited combat value.
A fifth brigade can be a helpful addition to a division, or a needless extra expense to supply where a 4 brigade division will do the job. I use them sparingly, if at all.
