Vallee,
for the dog collar I would go with a 3-4 oz. leather for smaller breeds (i.e. up to about 40lbs.) and with 5-6 oz. for the bigger guys. If you have an exceptionally strong "puller" (a Husky would come to mind), you can of course go with heavier stuff; although the sewing will get more difficult.
I don't have too much experience with tack (am still in the process of learning about it), so I would guess about 5-6 oz. for that as well.
Needle size: I use a 000 size harness needle for all but the finest of my sewing. I've had some people tell me that was way too big, but that size is what I started out with and I like the longer needle for better grip. Size 4 would be too small for my liking, but everyone's different.
Linen thread is the best choice. I use 5 ply linen thread from Campbell-Randall (www.campbell-randall.com, look under "Leather Supply", then "Thread", then "Linen") and run it through a pure beeswax cake a few times before threading. Look for a beekeeper in your area, the wax will be way cheaper than the little one ounce cakes from Tandy and others.
I don't know the Springfield groover; mine is the saddler's stitching groover from Tandy, which is great.
If you are just starting (as your sideline says) be prepared for some frustration with your sewing. Practice on scraps and some not-so-important projects for a while to get your "awl technique" (sounds impressive, but really isn't :-) )and stitching to look even. The Stohlman book is excellent, it's the only thing I used when I started out 15 years ago. The juggling of awl and needle in one hand is really awkward at first, but once you get the hang of it you'll be impressed how fast you can actually sew by hand.
Good luck and keep us posted!!
Black Dogg