I'm glad that you asked cuz I was going to answer it anyway
Most thru hikers and section hikers (at least I do it) have two bags - one for the colder shoulder seasons (fall and spring) and a lighter weight bag for summer. If you thru hike you're probably going to want two. I haven't done this but I certainly don't lug my 20 degree quilt when my 50 degree bag will do just fine in July. Also, pay attention to Rasty's "sleep system" comment - a lot of wisdom in those two words. My summer bag is technically a cheap, $20 sleeping bag liner from Amazon that weighs a bit over a pound. But I also factor my clothes into sleeping comfortably if summer temps dip into the 40s. During the winter I bring a 20 degree synthetic quilt (homemade) that sleeps two. My wife and I together add about 10 degree to that 20 degree quilt, plus the clothes I choose for cold weather and I'd be able to make it through a 0 degree night without any fear for me or my wife's life (though we wouldn't choose to do it if it was expected).
The clothes you take can augment your sleeping bag by 15 degrees or more. If you need to save money and are going in the colder months, get thick fleece jackets (front zip if available) from the second hand store. Bring flannel pajama bottoms for camp pants. Sure, you could buy smartwool or icebreakers for $100+ a garment and they are great. But when you're starting on a budget, fleece and thick flannel can keep you just as warm even if you look like the stay-puff marshmallow man
If I was in your shoes and just starting out, I would focus my time on fair weather/warm weather camping. You need less gear and it is cheaper. As you gain experience and learn what you like, move up to more challenging weather and more technical gear/clothing. I would also borrow as much equipment as I could from friends and try it out to learn what I liked and didn't like. That backpack might feel ok in the store but after 20 miles you might curse the day you bought it. Experience is worth its weight in gold.
Good luck! Enjoy the hiking.