Have you checked out the Vintage Knits book (by Sarah Dallas)?  Almost all the patterns in there are made with finer yarn.  The only drawback is that the patterns max out at a 36" finished bust, so if you are bustier than that you are kind of screwed.  But if you do fit in the size range the book covers definitely see if you can get your hands on a copy!

I'd agree that the neckline is usually not the biggest factor in determining difficulty.  A V-neck may be slightly easier since the neckline decreases are usually pretty much the same few rows over and over, BUT...that is not always the case.

Are there any particular patterns you're thinking of for your first pattern?

I think I'd make the soles red, the claws the darker grey, and the foot the lighter grey.  But that's just me

What exactly are you thinking of doing?  Do you want to convert a piece to fair isle all-over or just add a band of colorwork somewhere?

Adding colorwork into a piece is pretty easy--the one thing to check is that your motif will divide evenly into your garment.  i.e. if you have a motif that is 10 sts across, and your sweater is supposed to be 195 sts around, you'll need to add or subtract 5 sts to make the total sts an even multiple of 10.

I think the easiest thing is to just jump in and try a few.  Cables are one of those things that are sort of mysterious until you try them, and then they just click.  And then you'll want to add cables to everything

Seams can also give a piece more structure or stability sometimes.  However, unless you're making a VERY tailored piece or working with a yarn that stretches a lot, I don't think there's a real advantage, it's more a matter of personal preference (some people just prefer to make sweaters in pieces).

If you think you're going to be doing a lot of the patterns, maybe check out this leaflet:

http://www.interweave.com/knit/books/yarn_requirements.asp

As the title suggests, it really is very handy to have around when you are buying yarn!

Elann also has a guide on their website:  http://elann.com/faq.asp#estimates, although I have purchased according to their estimates in the past and I always come out with a LOT of extra yarn at the end, so I think you would probably be safe not bothering with, say, buying and extra ball just in case.

Do you know how to start a new ball of yarn in the middle of a row (i.e. you are knitting something that is all one color but you come to the end of your ball)?  Start the intarsia the same way, just pretend you have come to the end of your first ball and now you are starting your second.

But I was taught to start new same-color balls by working 2-3 stitches double-stranded (then drop the old and continue on with just the new). Doesn't intarsia make a clean change of color?

(I've been too cowardly to try color changes yet; I've dealt in texture, and let variegated yarns change my colors...    )

Oops, should have clarified that bit!  Yes, when you are changing between balls of the same color, knitting a couple of stitches with both balls of yarn helps to anchor the ends and keep things firm.  In intarsia you don't do that--just drop one and start with the other (so you make a clean color change).  It does mean that when you are knitting back across that first 2-color row there will be a couple of loose stitches at the color change, but it's only for one row.  And you can tighten them up when you go back to weave in ends.

You should absolutely try color changes, they are really not hard at all!  And working with different colors in one project is really fun.  Adding a little color is also a great way to use up small amounts of yarn that you may have leftover from other projects.

I thought I'd also chime in and mention that I did a little pictorial on afterthough thumbs on my blog a while ago: http://studiomarlowe.com/?p=53.

It is more the "after you have knit the waste yarn" part of things but maybe a photo would help you confirm what the knitting is supposed to look like after you've done it?  Hope it helps!

I was going to suggest Urchin also, since it's written for thick/thin yarn.  And it's a lovely hat.