As far as the cassette, as others have mentioned, just buy the whole cassette. Shimano does list part numbers for the individual sprockets, but undoubtedly if you buy 2 or 3 of them, you'll pay as much as the whole cassette.

Keep your old cassette. The 11-14 sprockets should be the same for the 11s 105 11-28 and 11-32, so if you wear them out, you can just re-use the old ones. Or... sell it on E-Bay.

I have heard that the Shimano derailleur ratings are a bit conservative. Just try out your current derailleur. You'll probably have to adjust the B-Screw a bit (all the way in).

If not, then perhaps a MTB derailleur. I've heard that the 11s shift ratios changed, at least with the road derailleurs, and apparently the M9000 derailleur isn't compatible with the road products.