Upgrading drive train for Cannondale Trail 6 I am planning to upgrade cassette, rear derailleur, chain rings and front derailleur of 2016 Cannondale Trail 6 
I am looking for 11-36 or 11-38 10 speed rear and 2 speed front.
Which model I need to select the cassette, derailleur and chain-ring set? I understand I have to change the shifters too.
Shimano XT range would be appropriate? My ride is offroad and onroad mixed daily and long rides (~60 KMs) weekends. 
Any 10 speed cassettes will fit on my 8 speed free hub? 
Or considering additional expenses, I am even considering 1X10 combination. with 33/32 T front with neat profile with no front derailleur . I feel when I ride with middle ring front and all the combinations on current 8 speed rear, is good. I guess my current middle ring could be 34. Will I miss the highway riding comfort with single ring in front? For lower spectrum I should be okay with 36T low end at rear
You will need to change the rear shifter, also best to install new cables (inners and outers) to get the most out of the upgrade. 

No need to stick with your existing brand. Microshift do a range of well priced products that would suit that bike very well. 

In Shimano, XT quality is probably spending too much for that bike, Deore is practically the same quality  the same just a bit heavier and a few less features (e.g. brake levers on Deore miss out on the tool-less reach adjust and miss the bite ajust)