Wouldn't count a cassette as something to resell; chains, cassettes & tires are really all consumables, I normally look at the 105/Ultegra range for this, but to keep cost down buy from Germany where it's a lot cheaper than the UK.

Shifters if you can keep in good condition really hold their value well, last year I sold a set of XTR 8 speed STI's for more than I paid for them, and I had originally bought them new about 10 years ago, the problem is you never know what is going to last, and also, as you are using a bike, how it will survive in use. There is another advantage to getting Dura Ace / XTR, which is 3 years as opposed to 2 for normal parts, never had to claim for any Shimano parts, but it probably will happen one day.