A “model” as in a building or vehicle, specifically excluding figures?
Must it be as true to scale as possible?
Complicated question, and one that I may not be interpreting the way you mean it. It must correspond with the scale of the other models and figures for the same project, yes. But by that, I mean the actual scale, not the nominal one. “True to scale” could be taken to mean that it must correspond with one of the established, traditional scales such as 1/300. If for instance my nominal 6mm infantry are actually 7mm, which is considerably bigger than 1/300, then I want the model scaled for 7mm as well, not true to 1/300 scale.
That being said, often the only way to be sure of a model’s scale is to buy it and compare it for oneself. If I’m interested in getting multiples of a model, or multiple models from one range, then I may buy one model as a test piece first, before I even know what scale it properly is. So in that sense, no, I don’t need it to be as true to scale as possible before I consider buying it, if it’s feasible and applicable to buy a test piece. As I said, complicated question.
If a model building, or in some cases a model vehicle, has been designed with a scaleless quality about it, then that may make me more likely to buy it.
Well, I’ll consider buying it even if it’s not cheap. Whether I’ll ultimately be able to fit it into my hobby budget is another matter, but that wasn’t the question.
Easy to assemble?
As long as it’s not fragile once assembled, then I don’t particularly mind buying a model that’s moderately, but not infernally, difficult to assemble. For instance, I buy Heavy Gear Blitz mechs, which I would define as moderately difficult to assemble (especially as I make sure to pin some of the joints), but I don’t terribly mind.
Quick delivery time?
I’m fairly neutral to this question. Quick delivery time is obviously a good thing, but that wasn’t the question.
Fit in with the rest of your stuff?
Look how you expect it to look?
I don’t really understand the question.
Quality in production. I don’t mind filling in a few air bubbles in a resin model, but more serious problems with quality such as misaligned moulds tend to be a dealbreaker.
Quality in design. For instance, I stay away from most laser-cut models because they’re too 2.5D (not 3D enough) for my tastes.