A few noob questions

[SMOKEU]

I would like to brew a strong cider using a Black Rock kit.

1. Is using dextrose better than using normal table sugar?
2. Does cider benefit from ageing for a few months like beer?
3. I want to make the alcohol content around the 8% ABV mark, should I add 3kg of sugar with the kit, ditch the standard yeast it comes with and add 2 packets of cider yeast from a homebrew shop?

 

[Kauai_Kahuna]

With three lbs of sugar you may even go higher.
Expect the cider to ferment down to less than 1.000 unless you stabilize it or crash chill it.
I personally like to add light brown or regular brown sugar, or table sugar with a little molasses, but I usually only add two lbs. If I want to further bump it I buy a few cans of concentrate. The rule of thumb is one can of tree top per gallon will rise the SG by 1.009.

Juice with a SG of 1.045 or so will give you an ABV of around 5.8%. (some juices may be higher).
Adding three lbs will get you around 9.5% ABV if you go to 0.998 like most ciders.
Depending on how high the ABV and dry you ferment it down to extends the ageing time. I have found it takes a very dry cider to take around 8 months to really kick a.., you could back sweeten, mix with soda, etc to match your taste.
Hightest has provided a spread sheet in the FAQ sticky at the top of this section, it is fun to play with and do all your what ifs.
As for changing out the yeast, I would not unless you suspect the yeast is not good. If you buy the kit I would just make it. Otherwise it may be cheaper just buying AJ, sugar from the store an going with that. BTW, that's what I do.
Hope this helps, and welcome to making cider.