Where does one start on doing a custom build ? I have a pretty good idea of the components that I want to use. Is it more of a matter of finding a really good gunsmith to assemble the rifle components ? Some of it’s a bit to technical for me but, I am sure a gunsmith can help with those questions or I can ask here

How difficult is it buying gun components out of the states , if I can’t find them in Canada ?
As others have mentioned what are you using it for, is it your only hunting rifle for everything, or is it a lightweight backpacking sheep hunting rig ?
Figure out what cartridge and what weight scoped do you want ? The cartridge choice will dictate weight to a point.
7 3/4 lbs to 8.5 lbs scoped rifle is a good all round weight to strive for.
I would stick to a stainless action, Defiance has many options and make a great action, Pure Precision SKLTN is a nice action as well with 75 deg bolt throw, defiance also has the 75 deg bolt throw option now.
Benchmark, Bartlein or IBI Barrels are all good
Hawkins Precision M5 bottom metal
Is excellent and make both BDL style or DMB
Triggertech Special triggers are excellent
Rifle Stocks, one thing I would stress for you to look at is a stock like the Gunwerks Clymer, Allterra Arms Carbon Hunter III or the Peak 44 Blacktooth, of course this is dependent on what you want in a stock.
With the Negative Comb they shoulder like a dream and absorb recoil unlike any stock I have ever used, they also have a big dish for the base of your thumb which makes the grip so comfortable and are a great compromise between a traditional grip and a vertical grip, if you can find one shoulder a Clymer Stock, not only are they great to shoot off the bench they are excellent for free/off hand shooting as well.
The Clymer and the Allterra Arms also have flush mounted picatinny rails up forward on the stock.
When you decide on your gunsmith be prepared for a 3-6 month wait time for a build, but also ask your smith if you can have the rifle by a desired date, if you want the rifle for Sept 10 deer hunt don’t drop the parts off Aug 15th as you will be disappointed, you’ll want your rifle a month before hunting so you can get to the range and shoot it and or develop a handload for it.
Don’t ever say to your gunsmith your not in a hurry for a build !!!!!