Wow! That's a lotta questions.
The most cost-effective fresh cut is 70% mince, ie 70% lean, 30% fat. eg Coles "3-star" which is $6 a kilo. Most butchers only sell lean mince these days, which will be more like 90% lean. Mince is versatile and cheap but it has to be fresh. We always have mince done some way on the night I've been to the supermarket.
Stewing cuts are good for winter but most of the summer here I don't want to run the oven or even a slow cooker. I use beef shin a lot, as we like the marrow too and the bone and gelatin add minerals and richness to the broth.
For lamb I mostly use loin chops for frying, shoulder for slow-roasting, or diced lamb for curries.
I prefer not to buy in bulk because I like my meat dry. When it's been in cryovac it comes out kind of slimy, and the mince cooks dry as it's bled in the packaging. But you can save a heap of money that way if you don't mind the cryovac. eg my butcher charges $6.99 per kilo for a whole side of grass-fed beef, cut however you want it. For sides he uses small carcasses, so it's about 100kg. If you have a big freezer and a spare $700 or so, you could save a bundle. In a good chest freezer it should be fine for six months, which is about how long most families would take to eat that much beef, although on paleo you might go through it in four. Be aware that on a full side there is very little prime steak and a LOT of mince or stewing cuts.
My butcher gives me fat for free because I'm a regular. I think most butchers charge around $3 a kilo for it. If they leave the fat on the cut you'll pay more than that. Ask your butcher which day they break down beef carcasses - that's when they'll have fat and bones going begging - and make that one of your regular butchery days.
I have a slow cooker but actually I don't use it that much. I tend to find my Le Creuset casseroles more convenient because they can go straight from the stove-top into the oven so there's less washing up. But I use the sloww cooker if I need to put something on way in advance.
For organ meats there will be particular days of the week the butcher gets them in fresh. You do want them fresh so buy them that day. Some things you might have to order specially - it will depend on the butcher. Most butchers will stock lamb's fry at least, otherwise it depends on where you live, eg if there's a large immigrant community where you live they will have a better range of offal. If you get to big markets like Vic markets you can get just about anything. Offal is also a great buy, and packed with nutrients. I pay $2.50 each for lamb's fry, about $4 a kilo for kidneys. I don't bother with heart as it's basically just muscle meat and it's a pain to prepare. If you can get lungs ("lights") they are good in home-made sausages and usually insanely cheap as it's regarded as pet food. Sweetbreads are yummy if you can get them.
