Now we know it's an EC34 headset, stack height is the only thing to be aware of if you change model but it's rarely an issue with threadless setups unless you've cut the fork right down so there are no spacers left. You can generally just add or remove spacers to suit.
On threaded forks, stack height can be a nightmare. Some 90s bikes had their steerer cut very short to suit a Shimano headset with very low stack height.
No. The steerer tube has to stick up above the combination of headtube plus headset stack depth. You need enough excess to clamp a stem onto it (maybe up to 50mm), and sensibly a thin (2.5mm) spacer above and below.
The Brand X headset you linked to earlier has a stack height of ~29mm.
So the steerer tube of the e-bay fork you linked to @ 185mm is no where near long enough.
Look for a steerer tube of minimum 150 +30 + 50 + 5 (mm).
200mm is a really short steerer for a new fork. They're usually super-long so they can be made in one size and cut down by the installer.
Bear in mind that many new 1 1/8" steel forks will need the steerer tube's crown race seat cutting to an accurate 30.0mm diameter, with a perfectly flat and perpendicular base. This is not always done at the factory. Any decent bike shop will have the tool.