Any question answered...

What ultegra rear 10spd mech do I need for the commuto x bike.

I have DA 7800 shifters, an ultegra 6750 50/34 crankset and an 11/28 cassette in the parts bin.

Most of the time the bike will be used on the road and for carrying stuff. But there may be times when I want to drop the gearing a little lower. It looks like changing to some smaller chainrings might be an option.

So what rear mech should I go for, and what options do I have for chainrings for offroad or carrying loads up hills etc?

I am terrible at working this stuff out, I think 50 + 28 - (34 + 11) indicates I need a rear mech that can take up 33 t, but if I am to run a lower gearing, what are the options, and is there a rear mech that can do it all for me?

Should I just go for the Ultegra medium cage mech (i can't find an ultegra long cage) or should I look at a 105 or is there another mech I should consider?

6750 cranks won't take rings smaller than 34
Shimnao road RDs won't go bigger than 27t (well, they're not supposed to; you can sometimes get away with a few more, 28 will be no problem)
The RD-6600-GS will work with what you have, you can't do properly low gears with Shimano road parts without going to a triple front.