Something that I noticed when putting together my capsule wardrobe was that I tended to keep the best of what I had. These are things that I've worn a lot; favourites and higher quality pieces that I have maybe spent a little more on over the years. In this age of very fast fashion and ever changing trends, it's so easy to throw away damaged or well worn items and just replace them; and although there comes a times when you will have to replace a favourite (impossible, though it is; I'm still trying to replace a pair of woven leather loafers which fell apart about 4 years ago and have had no luck!) if you can keep them looking their best for as long as you can, you'll be able to enjoy the longevity that they provide.
Knitwear is a particularly contentious topic because it's quite difficult to keep looking nice. I have a couple of cashmere jumpers, some mohair mix pieces and some cotton/polyester blends. You'll find that the more man-made a jumper is, the less likely it is to pill and wear, so should you want a jumper you can wear hard, you'll probably be better off shopping from a base of less natural fibres. However if you prefer a more luxurious garment, you can buy into wools, cashmere, mohair, angora and silk mixes.
Cashmere can, for the most part, be hand washed, and if you use a gentle detergent it will nicely refresh it. Dry it flat, rolling with a towel to squeeze out most of the moisture, and then allow to air dry. Don't hang it up to dry or you'll end up with ape arms where the weight of the water will draw the fabric out of shape! You can fold it with tissue inside to soak up any residue of moisture and it can be packed away when completely dry.
Something I learned when working in retail is that as a rule of thumb, the more expensive a garment, the greater the likelihood of pilling. Pilling is that incredibly annoying ability that nice jumpers or cardigans have of going all bobbly across areas that receive friction from movement, for example, under the arms, or across the chest if you wear an across body bag. High quality cashmere and wool blend knitwear uses the longest yarn to ensure the softest handle, so these yarns, by default, are more likely to pull and roll up to make those annoying bobbles. Invest in a cashmere comb for easy debobbling which won't hurt the garment, you can pick one up for a few pounds and you'll often find that higher end stores will give you one with your purchase. If you spend a sum on an item, you're going to want to keep it looking as pristine as possible for as long as you can.
Most advice around cashmere storage involves folding it down, to prevent it stretching. I have to admit that I hang my cashmere jumpers on padded hangers in my wardrobe, as I don't have much space for folded storage. I don't find that hanging them is detrimental to the shape of the jumper but that's probably because the pieces I have are quite lightweight. If you were to have a very thick or chunky knit I expect folding it would be the best bet! To avoid any moths munching holes in your garments, you can use mothballs or bunches of lavender to detract them.