We work with small and medium sized businesses every day. That is what we do. I would STRONGLY SUGGEST taking some business training, like Next Level or Fast Track BEFORE you decide to jump. Your Small Business Development Center or community college will know what these classes are and when they are offered. Basically, they are business 101 and 201 combined, 13 weeks, 3 hours per week. If you do the homework and really pay attention, you will be golden. My experience having taught these courses for several years is that the ones who really do well are the ones that "buy into the philosophy" and do the homework.
You will end up with a rich business plan if you do this right. You really need a business plan before you start. Otherwise you will flounder and possibly look at the window most days, wondering why the phone is not ringing.
Depending upon what you are going to do, you can find a lot of used equipment that is perfectly serviceable. Of course, I would not buy any computer/software/printer products pre owned. I would buy office furniture, etc preowned.
There are some great books on how to start a legal practice on the web. Read those. The domain, engineering v law is different, however, the paradigm is exactly the same.
If you take a partner, make damn sure that you have a partnership agreement drawn up by a lawyer. Somebody has to be the 51% partner-- ideally, you will find a partner that has complimentary skills, one guy likes going out and getting the work, the other guy likes doing the work. If both guys like doing the work and nobody is out selling, the days can get mighty long and you will go hungry.
NOT use legal zoom or some other web based service. Why? Well, depending upon your situation, the business structure may be different.
We always recommend a good attorney, accountant and consultant (like us!!) before you start. Yeah, you are going to spend some $$ up front, however, rest assured that they will be nothing like you will spend if you get yourself in a bind and have to get out.
I have enough stories to curl your hair and if you have curly hair, it will look like a fuzz ball!!!!!!
Go slow and think this through very carefully. Employees and supervision are another challenge. Remember, the troops have to eat first before you eat--- ie, you have to pay them first, then you. This can be a real challenge.
Think about this real hard. It is worth doing, but worth doing well.