Kv is a determination of prop size and cell count.
a 600 watt motor with three cells might be 3000kv and have a 4.1x4.1 prop with would barely move a 6lb airframe. a 350 watt motor around 400kv running on three cells could have 180oz of thrust on three cells, but would require a 16" prop. there are quite a few variables, and they all matter.
you need to properly match all needs for the plane.
id reccomend using headsuprc at the very least as a guideline for what motors to look at. anything from a .25-.60 size motor may be appropreate for your plane. compare the cell counts of the motors to what you have and the size prop that would best fit the plane.
you moght want a 10" prop for ground clerance, or a 14" for lower watts and same thrust on the same motor, again depending on cell count.
i learned far more from reading on headsup site than almost anything else. another perk, send an email or call jeff at heads up, he will give you a recommendation, and you will not.be disapointed.with what you will end up with.
slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
Kv, while an absolutely critical part of the system, is actually the item one should choose last.
Decide your peak power requirement based on the weight of the model and how you want to fly it
Pick a preferred cell count (voltage) and pack capacity for how to deliver the power
Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate
Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.
So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.
What's a cretin? Not familiar with the airplane. Wing Span? High wing? Low wing? LG?
I have a few 5# + models . Using a 32 on the Escapade but a 46 on the HK Beaver and Phoenix EMB 312 Tucano and GP Piper Cherokee.
+1 on Headsup RC as a quick reference to Motors, lipo voltage, Watts generated, amp draw by prop size, etc.
Sometimes getting the perfect combo takes a little experimentation.