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)
The latest Shimano Specifications Chart reveals which model numbers fit your parameters. Be aware that there are other model numbers which would work but are unlisted in this link as they are older models. These can be accessed by using the Shimano Product Info Archive and then selecting the appropriate PDF file. Anyway, in general, double front Chainwheels come sized with 40-28T/38-28T/36-26T/34-24T ring combos and the top low sprocket on a Shimano 10 speed cassette is 42T which should be used in combination with a 10 speed rear derailleur having a GS sized cage. Your current freehub will accommodate a ten speed cassette. Additional considerations are that you will need a ten speed chain and possibly a new bottom bracket compatable with your choice of chainwheel.
Brand new components will run into the hundreds of dollars (probably not more than $300 at time of this writing in October, 2019). Higher tier XT components will be more expensive than SLX or Deore will be of course. Looking for used components either online or at a nearby bicycle coop will save some money. There are some incredible values to be found out there, I can personally attest to that. Along with the cost of componentry, there is a good deal of labor involved in a drivetrain upgrade. Even if it's your own time and labor you intend to put into this project, it's worth considering. I try not to overlook the time and effort required to find the appropriate parts and make sure they're compatible.
All of these "considerations" are why many will advise that shopping for a bike that is already equipped with what you seek ends up being a better value than upgrading expensive components.