This is a description of how my railroad model building business. fits into the custom assembly of kits.
There are many very qualified people who will build your kit for you. I get involved if you want custom work that is beyond the person who is building the kit; I am happy to work with your builder on this.
A word to the wise about kits. Many times parts are missing from second hand kits. If the kit is no longer in production, I strongly recommend that you or I inventory the kit before you commit to the job. The time for you or me to locate missing parts, or the cost impact of having me fabricate them needs to be understood before the job is started.
Also, some kits are of very poor quality, and the level of work involved in their assembly can approach that of scratch building. I recommend that you look at a kit that has been built up by someone other than the manufacturer or their suppliers; determine whether its appearance is acceptable to you, and ask the builder about assembly time and difficulty.