Quickest way to suck the joy out of your model building is to make it a business. Never do what you enjoy as a hobby as a living. As the guys have said most of your work will be prototyping and architectural. Most firms will bid out unless they have a sole source that they keep feeding work to, but its easy to become overwhelmed. This means you'll have to bid against 2 or three other firms or builders for the job. Low man gets the job. While not always the case because you do get what you pay for, it is pretty common.
Toughen up. I mean really toughen up. Heart of ice and skin of steel. There is nothing more crushing than to have your hard earned work trashed by a client because his wife hates the color scheme (why they would be interested in the color of a water recyclng plant is beyond me) or the technical director of a theatre or film project has umpteen changes and your build doesn't match his vision. Wait till your first client meeting and they tell you, "boy this sucks, my 3 year old son did better than this with his tinkertoys and leggos" despite what your compatriots, other modelbuilders and artists have lavished praise upon. Modelbuilding is likened to vanity work. Someone elses vanity and vision of their project, despite that you may have built it from whatever plans they gave you.
Other problems will be firms that subcontracted you lost their bid, and the money to pay you with. The project managers wife wants a change made, and you still have to make the deadline. How to get it to the site, office, museum, movie set. etc. A guy I worked with sat on a huge model for a production company for a month waiting for them to pick it up, they were waiting for him to deliver it. It was 8' wide and he had no way to get it the 300 miles to the studio. Hadn't included it in his bid and they hadn't included it in the rfb.
I'm not trying to disuade you from your desire, just letting you in on the secret handshake stuff that no one tells you and you normally have to find out by yourself.
My advice would be to hire on with a model shop and work with them for a while and see if its something you really want to do. While you're taking fabrication classes, or industrial design classes see if they offer an intern program with a shop. You'll learn more that way than you would 10 years of school.
Good luck with it. The payoff is that you will be doing what you want to do, when you want to do it and look forward to going to work in the morning rather than enduring your workday till you get home.
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"