When you make a custom, you're basically making an entirely new wolf and then choosing to apply that coat to an existing wolf or not. When you import a wolf, the price at the bottom is the price it would cost to make a wolf identical to that one. If you make something like a 59R AC custom, then apply it to a wolf that shows the cost as 10 apples when imported, you won't be paying the amount for the 59R AC and the existing wolf added together. Basically, make the design you want, then look at the cost. That cost will stay the same whether you add it to an existing wolf or not.
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I'm pretty sure vitals and weight are randomized, similarly to how starter wolves have random vitals and weights. If you create a new wolf, it has a 0% chance of having a boost or defect. However, if you add the custom design to an existing wolf, that wolf will keep its boost/defect if it has one.
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I'm not really sure what to tell you for this part, since it's mostly preference. There aren't many pros and cons to making a custom, but the price could probably be considered a con, because they get expensive very quickly. If you plan on making the wolf a stub/breeder, 59R AC is probably best, since it's the highest you can go with common markings and pups are likely to have fairly good rarities too, but it's up to you and what your budget is. I don't think there's such a thing as a 'good' custom or a 'bad' custom, but (again, for studs/breeders) it seems like HW Heroes, sometimes with a boost or defect, are the wolves that are most commonly customized.
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Hopefully this makes sense. Customs are mostly personal preference, so there's no wrong way to make one, but a lot of people do look for specific things when it comes to studs, so that could change a few things if that's what you're planning to do with your custom. Sorry if any of this comes off as harsh somehow. Please let me know if this is confusing or if you want me to explain something more.
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