Leave more than 3mm above the stem at this point - until you've ridden the bike, with the bars and brifters in the desired positions, it's going to be hard to fine-tune your stem position - you want to leave yourself as much room for adjustment as possible. I don't know what you mean by measuring the steerer from the bottom of the steerer tube - IMO this isn't a great way to determine the cut point - just get the stem into the approximate position, with the requisite (if any) spacers underneath, mark the position of the stem top on your steerer, and base everything off that mark - cut 3mm lower than that mark if you know your stem is in the ideal position (although I don't know how you could make that determination if you haven't ridden the bike), or cut ~1" higher than that mark if you haven't spent any time on the bike identifying the ideal stem height.
I've been pretty happy with Kobalt 1/2" and 3/8" click wrenches from Lowes - well made and seemingly pretty consistent, but the trick with torque wrenches is to baby them a little - don't knock them around, de-tension them when not in use, and keep them in their cases. On the bike, I use the 3/8" for installing the BB, crank and cassette. For low-torque applications (pretty much everything else), I have a 3/8" click-type (brand u/k) graduated in inlb, but the range was a little high for CF component applications, so I went with a beam-type 1/4" wrench I found on Amazon - can't remember the brand, but it cost ~$20. There's not a lot that can go wrong with a beam-type. Park stuff is OK, but IMO you pay a lot for what you get (it's that "blue surcharge"). Unless it's an exclusively bike tool (i.e., with no other application), I usually look elsewhere. $108 for a low range torque wrench seems like a lot (although that might be just Competitive Cyclist pricing - they're not the cheapest, and you might find the same wrench elsewhere for less). My 50-250 ftlb Kobalt, which is well made and has been great, cost ~$90.