MTB Conversion For Lighter Commuting
Its not a light bike, even by MTB standards, and will never be a great road bike as it will always be heavy. 

Start with the easy and relatively cheap stuff - tires and tubes. Dump excess rolling weight by getting light tires an even going tubeless. The next biggest bang for your $$$ will be replacing the fork. You should be able to pick up a second hand fork for next to nothing - just watch the geometry. If you do this, canti brakes on the front is likely worth doing. 

Replacing wheels is probably not worth the effort, you will shave off  a few hundred grams (admit-ably rolling weight), but it will cost big money.   

Look very hard at the costs of any proposals. My guess is that for what you are proposing, a cheaper option that will give you a better outcome would be to buy a second hand bike that does what you want, rather than trying to make your current bike into something it is not.  You may have access to cheap parts, but in that case you probably have access t cheap bikes.