I don't have all the answers, but I will give you some of my opinions here.
1. Hire an IT department of people that is familure with linux and can solve the problems one might incounter. It doesn't have to be a large staff, it just needs to have good people on it. Afterall, Linux is a constantly changing entity and you need somebody there to upgrade the programs, apply the security patches when they are availible and overall act as the system administrator.
2. Linux can be a cheaper way to go. It does, in general, require less work then a Windows platform, but not neccesarily any less work then a proprietary unix platform to maintane. It will run on cheap hardware, and much of the software is free. Remote administration and setup is possible with tools like SSH, but when the crap really hits the fan you will need somebody on site. While this may never happen, it is always smart to plan for the worst.
3. This is where people seam to not understand what some things really mean. First off, Open does not imply Free. You can have Open Source software that is not availible free of charge. Secondly, Free does not imply Open. Software that is closed source can also be free. Third, Linux the OS is Open and Free, Linux programs do not have to be either Open, or Free. Take Opera for example. It is a free web browser that is closed source. VMWare is both closed source and not free. There is no reason why somebody could not make a CAD program for Linux that was closed source and charge money for it. The GPL that linux is released under does have restrictions on what can be done as far as modification. The major catch being any modification made to GPL code must be made availible if a program is distributed using the new code. There are many other restrictions as well.
I am unaware of any CAD programs that run under linux but I know companies have started using Linux as a replacement for IRIX workstations to do graphics so I am sure something exsists out there that will do CAD under linux.