I have heard of people doing CAD work for hire. Not sure how you'd go about doing it short of networking with engineers and architects, which takes time and social energy.
The software cost seems extreme, but there may be smaller packages that you can use that would provide most of what you need without anything super fancy, like, I don't know, modeling and simulation? In other words, you do the grunt work of placing the vertices, and they run their sims on it.
The following would be one reasonable way to pursue it:
1. Determine a minimal setup first.
2. If you have engineer or architect friends, let them know you're starting a business doing contract CAD work. This is important from a branding perspective; the difference between "part-time contract CAD business" and "guy on weekend who might could help out" is huge.
3. BE AVAILABLE. Freelancing takes a certain amount of willingness to drop "everything" to help out a customer. If you're not available for the first couple opportunities, they won't bother with future opportunities.
4. Practice good business sense. Always return calls, try to always say, "yes," in some form or other, build up your network of providers, be reliable. Remember, you're selling trust that you can do the job efficiently by a certain time. Also, understand your day-job's moonlighting policy. If it's bad, find another job. :P
As far as income goes, you'd probably just report it on 1040 Schedule-C as self-employment income. You'd pay the full 13.whatever% for FICA and Medicaid, plus income tax.
If you make a lot, then you should pay estimated payments quarterly. I believe you're penalized if you don't pay 90% of the taxes you end up owing by the end of the year. Or something egregiously complex like that.
