I personally like to use 9 inf/3 art with an Engineer company as the basic cheap division.
Then, if possible, I'd replace the Infantry with Mountaineers and Marines, the reason being that those troops types edit: have significant bonuses over regular infantry in certain types of terrain or when crossing rivers and don't cost that much more (40 Infantry Eq. more per Mountaineer, 50 more per Marine). You can decide how much of each to use based on where you're fighting, so for example if you're going to be crossing a lot of rivers but not so many mountains, maybe use 5 marines and 4 mountaineers; but if you're going to be fighting in the hills and mountains and not crossing as many rivers, then you can use 6 mountaineers and 3 marines.
Then after that if I can afford it, I'd add a Light Tank to the division to add some Breakthrough, which is useful. This can be replaced with a Medium Tank II later on. I'd also add a Maintenance Company when doing this because it helps a lot.
Next, over the course of the game, I'd gradually replace the 3 line Artillery with Medium Self-Propelled Artillery. You could also add other support companies like a Field Hospital or Anti-Air, but personally I don't think Support Artillery is worth it except for paratroopers, who obviously can't have line artillery and still need Art of some kind.
I'm by no means an expert but this is what I usually do and it tends to melt AI divisions like butter. Even for minors that can't afford the M-SPG or lots of tanks, the basic cheap 5 Mnt/4 Mar/3 Art/Eng setup is pretty good in my experience.
Oh, and you're probably going to want some faster divisions to do encirclements or beeline to certain provinces or whatever. Some people say using Cavalry is just fine and more cost-effective than using Motorized/Mechanized/Armor divisions, but personally I prefer Mot/Mech if I can manage it. For super poor countries of course Cav will have to do. But to put it simply, I don't think "mobile" and "armored" divisions need to be a separate thing, I usually just use a little armor in my generic division for some breakthrough and hardness, and then have some mobile divisions which also include armor. However I suppose if you want your divisions to have a very high Armor stat you can stick a single Heavy Tank in there.