1) Certainly possible, if your line of work requires you to use this software at an intensive professional level then a workstation PC with a Quadro/ FireGL GPU would be best suited to this.
2) Impossible. All modern systems are single user.
If you wish to play games on it as well then a workstation system will NOT suit you. Workstation hardware cannot be guaranteed to work with 3D gaming applications.
3) Possible, pre-build systems can have quite extensive upgrade options with only case layout and PSU limiting them.