The C package really sets you up pretty well for getting started and if you add it up, that is a pretty big savings over buying at all separately. That is exactly what I started out with as well. You are smart to go for the 4400, I've never needed the whole 17" but I have definitely had times where I would have regretted being stuck with the shorter 8" lathe. It also comes standard with some nice features not included on the 4000.
As far as attachments they are kind of like Pokemon, gotta get them all.
Seriously you will eventually spend more on tooling than you paid for the lathe., it is not hard to do and is very normal.
Really, you will be in a pretty solid place with that package until you have a better idea of what you like doing.
The only things I would suggest you consider with your initial purchase, I'll start off with some highly recommended items, and then provide some other things worth considering, but can wait until you have a better idea if you will use them. All dependent on budget, if you have the money to spend these could be nice to have, but if the C package is already pushing things you don't really need anything more than you are getting in that package.
High priority
Upgrade to the 3.1" 3 jaw chuck, it is an option when you order the lathe and well worth doing.
Get some additional rocker tool posts, at least 2 more.
I have an OXA quick change tool post and while it can be useful it really is a bit large for the Sherline and I'm probably going to transfer mine to a larger lathe. If you get a couple extra rocker tool posts you can leave them set up and swap them easily, not quite as fast, but not real slow either. Later on you can make your own QCTP or buy one that is really made to fit on the Sherline lathe. Buying one made to fit a Sherline is expensive, but there are several DIY options out there when you are ready.
The cutters included in the package are good enough to start with, but I'd order some extra HSS blanks and learn to make your own. There is a good thread here on grinding HSS lathe bits. They are pretty cheap so I'd get a 5 pack of the 1/4" blanks at a minimum to practice with and make a few different tools, 10 would be even better. Here is the thread on grinding tools. It is a long thread but very worthwhile.
http://www.hobby-machinist.com/threads/models-for-grinding-hss-lathe-tools.62111/
Get some spotting drills, at a minimum get one 1/4" as that can do the job for most holes you would drill on the lathe. Sherline does not sell these so you will have to buy elsewhere. I got a set of 3 with 1/8", 3/16" and 1/4" for less than $20 off of ebay.
A lot of people just use a center drill but a spotting drill is the correct tool for starting a hole. Pretty cheap, just make sure you get one to match your drill bits (120 degree if you have the common 118 degree bits, 140 degree if you have 135 degree drill bits).
I'd also pick up another, better quality T handle 5/32" Allen wrench or two. The one Sherline includes is ok, but I like the Eklind T handle wrenches a lot better. You can buy singles a lot of places, Amazon, Ebay, local hardware store etc. You will use this a lot with the tool post so it is nice to have one dedicated to that as well as having a handy spare in case you misplace it.
A good light either desk mounted or overhead (track lighting with spots over your work area work well). No such thing as too much light.
Not immediately essential, but I would get an independent 4 jaw chuck (not the self centering 4 jaw). This allows better precision than the 3 jaw, as well as the ability to hold odd shaped pieces.
Other tooling to consider. These are things you may find a use for but are more specialized, some will find them essential, others they will just gather dust.
Radius cutting attachment - Handy if you want to make half round or ball shaped parts. You can do simple curved parts that look good just using a file, but the radius cutter lets you make repeatable shapes as well as nice even half round to full ball parts. You say you want to make knobs, so you might find this useful.
Knurling tool holder - If you want to add knurls you may find this useful.
Follower rest - The C package includes a steady rest for long parts, but does not include a follower rest. This can be useful for turning thin parts as it provides support right next to the cutting tool and moves along with it.
A cover, doesn't have to be the one Sherline sells, but it is a good idea to use a cover to keep dirt and dust off the lathe, particularly if it is around woodworking.