Bachus & Fran, in addition you need to find a suitable sampler that can switch sounds easily & quickly from one to another, e.g., piano to guitar, etc. To do this you may have to suffer some loss in quality by going with wavetables that can accommodate this, e.g., Bandstand, SGM180, etc. I use higher quality VST and samples for my lead instruments, e.g., kontakt & Garritan GPO or JABB, etc.
I can't comment on all the combinations which Bachus refers to but likely most of it can be done. Like anything else you will need to make some compromises as determined by the hardware, e.g., computers, hard drives, controller keyboards and so on. The good news is that most of this equipment and associated software exceeds by far what you would find in hardware based arranger/workstations - it is more efficeint, effective and reliable....mostly it simply sounds so much better. I believe the hardware based solutions are good enough for live performances. Those who do not wish to invest the time to setup a software based solution would be well advised to stay with the hardware based solution.
I have been at this for years and will never go back to hardware based solutions...unless there is no other alternative.