Jonathan,
Good luck on your venture.
I've been building on a contract basis for many years - not as a sole source of income, but just to help pay the bills and pay for tools and materials. I do a lot of resin casting and master fabrication - it can be fairly lucrative if you do it right.
Pretty much all my work has come from talking to people at hobby shops, contests, and word of mouth. I've gone to Jaguar Car CLub and Ferarri Car Club meetings with a number of built models and boxes of unbuilt models - and walked away with a pocket full on money from the built cars, and lots of orders for personalised models. Other than that, I've never really advertised or actively "marketed" myself other than face to face. Many of the referrals are from friends.
I usually charge x2 the purchase price of the kit for a simple stock build - up to x4 if there's scratchbuilding or modification required. If the buyer is filthy stinking rich, I may charge a little more, but give the model a little extra detailing.
Keep one thing in mind - if you're building to a deadline, remember that you're NOT building for a contest. Most everyone you build a model for CAN'T do it themselves - they probably have NO IDEA what the difference between a good model and a great model is - you will be your own harshest critic which can hobble you - don't let it bog you down. 99% of the people you build for will think their newly built model is the best thing they've ever seen, even if you think it's not your best work.
Also, life often gets in the way of model builds, so I'm quite up front with my clients and tell them that sometimes their model will take a while to get built - they've never been disappointed with the final results, and they don't mind waiting.
Try not to take on too many builds at one time - you'll only end up stressing yerself because you'll feel the need to be in three places at once. I know, it's hard to turn down work, but sometimes you'll find it necessary. Be realistic - don't give a delivery date of 1 week when there's 2 weeks worth of work to do.
Well, good luck with your new venture.