Guess the first step is to create yourself a portfolio of your modeling accomplishments...ie awards, features in publications etc. Not to be offensive but there are a lot of modelers out there who think they are good, but in reality they'd have to offer a cash incentive to get someone to actually take them home with them.
There are jobs for modelers in the industrial sector. In fact there are programs at various technical colleges that offer such courses. I know there is one such program in Green Bay, WI and a fellow club member is following his dream and attending now.
Build scale models for personal collections is hit and miss. Usually once someone finds a customer or a customer finds a builder they stick together. There are a lot of customers out there who want models but are not willing to pay a realistic amount to acquire them.
Modeling full time as a business for many has destroyed their passion for the hobby. When fun becomes a J O B things change in a hurry. I'm not trying to sway you from your dreams, but unless you have the ability, are self motivated, very dedicated, and most importantly have the means to support yourself for prolonged "dry" periods you might just be able to do this.
Businesses start and fail everyday. You cannot run it as a hobby interest if you want it to be successful. Those who do it as a sideline, do so with the understanding it isn't their primarly source of income and allocate time accordingly.
In this time of economic uncertainty, finding customers with disposably income who are willing to pay a fair price is getting harder. Starting a business is also risky too. If you want to do it, build that portfolio and market yourself effectively. Some start selling their works on eBay...it is a good way to reach a large market. Just don't forget the project isn't done until the model is in the hands of the customer...building a customer well build model can result in disaster and nothing to show for it after a shipping accident. Another risk to consider.
Weigh the facts and go from there. If I can help, I'll assist in any way I can.
Gerald