Personally, I perfer building kits that I'm interested in. I would not like to build a Dinosaur for someone lets say or medieval figures for example. I have no interest in those subjects. I could not see myself making a good job on such models. If a friend ask me to make a model for him and I like the subject, I'll do it with no hesitation but I doubt that I could do any money with it. Lets calculate this: the kit $40, aftermarket stuff $30, paint $10, x-acto blades $2, glue $3, time ???????
I think if you want to do this you must be aware of prices, time it takes to build, etc... to be able to come up with a realistic price.
If I would do it for a friend, I would only charge for the material and not for the time. I spent almost 40 hours on my last Tiger I late armor.
No matter where you go ....... there you are.