A water cooled spindle is a good option, on my opinion for starters I guess dont use the chinese ones, for it`s going to be prone to abuse because you are still learning how to cut/engrave/carve stage, it tends to fail when you are still in the learning curve which causes you additional headache at least the branded ones are resistant to abuse, by and by when you get sharp enough (strength of material, cutter properties, speeds and feeds) I guess it`s now safe to buy the chinese ones for it`s cheap, specially if you are adding more machines for mass production purposes
Chinese spindle are reliable in terms of longevity, been using chinese spindles for at least 3 years now (800-1500W) just dont over stress it, the problem arises when you are cutting too deep, too low RPM, dull cutter, etc.
I just took a glance on your picture, are you using a ball screw? if it`s not then you will have inconsistencies due to the backlash, a 0.3mm backlash will have enough to offset your spindle running a code loop at several times, for example if you zero in a certain coordinate then try running a code loop several times, I guess after it zeroed it will eventually be offset zero than the original position.