Roasting the bones will give you a darker brown stock than using the raw bones. To roast the bones, just stick them in an oven on high heat, around 450 for about 45 minutes, or until they are a nice golden caramelized color. Though you will want to make sure to keep an eye on them the first time, I'd check every 5 minutes after half an hour. Roasting the veggies with the bones will also add a slightly sweeter roasted flavor. It's like the difference of putting slices of raw onions on a burger compared to caramelized onions. You get a slightly sweeter, richer roasted flavor. Though it does turn down some of the other flavor notes, it's up to you which you'd rather have. Without roasting, you'd have a clearer "white" stock.
If you are using the butter as a base to roast the bones, I would set it aside and go with a higher smoke point oil like a peanut or corn oil. The low smoke point of the butter could leave a bitter, slightly burned flavor, especially with the longer roasting times.
I would leave the seasoning for the stock. Most spices will burn at a lower temperature, and salting the bones before you make your stock, could make your stock overly salty. It's easier to add more toward the end, than try to figure out what to do with salty stock.
I hope that helps!
Chicken stock (or any stock) is made from bones; that generally means backs, necks, and carcasses - which are generally the cheapest "cuts" because they have almost no meat and aren't useful for much else. Sometimes, if you approach a butcher with high turnaround, you can get them at a steep discount, well under $0.50/kg, because they just end up throwing most of it out.
If you add any meat whatsoever then you are essentially making a broth, not a stock. I repeat, stock is just bones, simmered long enough to denature all or most of the collagen, usually no less than 8 hours.
I find that most people (myself included) prefer brown stocks, so roast them first. You can sauté, but it's not as effective as a slower roast because the heat doesn't really penetrate far inside the bones. It also helps to cleave the bones before simmering (after roasting) to expose the marrow; it makes for easier extraction, although some people don't like the marrow flavour as much.
The least expensive way to add more flavour is probably to add the dry stock mix or bouillon cubes filled with salt, MSG, and other flavour enhancers. Of course it won't really taste like a homemade stock if you do that. If you want a good stock then the traditional way to flavour it, which is also very inexpensive, is to use a mirepoix, which is just a 2:1:1 ratio of chopped onions, carrots, and celery. You add this about 1-2 hours before the stock is done and remove it before clarifying.
Going up the cost/taste scale is a bouquet garni, which, although the herbs that go into it can get fairly pricey, the amount you use is minuscule, so it costs practically nothing if you have other uses for the remainder of the herbs.
Don't even think about trying to put "expensive cuts" of meat into a stock to improve its flavour. You'll just be wasting the meat, which loses most of its flavour when boiled/simmered, and turning your stock into an unsalted broth, which makes it less versatile and generally perceived as blander.
Now if you want to make a soup with better cuts of meat, that's great; in that case, the texture matters, and I'll occasionally poach some boneless thigh or breast meat to use in a chicken soup (made from the aforementioned stock) for the sake of convenience. But I emphasize that this is done after the stock is made. The soup will also have other ingredients added such as chopped vegetables and of course plenty of salt.
Stock isn't supposed to have an exceptionally complex flavour, that's why it's called "stock" - it forms the base of other dishes such as soups, broths, or anything that calls for flavourful liquid (like a risotto). Focus on making a good clear stock from just bones and mirepoix; it gives the best result because you tweak it later according to how you use it, and it costs you almost nothing besides the energy it takes to keep the pot heated.
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I never worry about this. As your stock simmers, the joints, muscle and connective tissue break down and eventually they'll sink in. Sometimes adding a bit of vinegar to your stock first helps with this. Until then, just stir the stock and move the bones around occasionally.