Single User: Any WINDOWS. Actually you can start Multiple sessions from a single user. Your Limitation is One (1) Key Board. Which means that if it takes more than a few minutes to produce statements etc. those processes can run at the same time as your interactive use.
Multiple users Same Location: Any New WINDOWS with Maximum Memory & Solid State Disk For a Peer to Peer Network for up to 3 - 4 Users, or any WINDOWS, LINUX, or UNIX Server for More than 4 Users.
For Multiple users & Remote Access: Daily on demand usage: WINDOWS/Server using Remote Terminal Services (RTS). RTS will start an independent session that will not impact any Workstation or Server Desktop. RTS will also allow Security that will limit access to specific PROSERVE programs only.
Occasional or After Hours Remote Access: Any version of WINDOWS using PCAnywhere, RealVNC, Remote Administrator, GOTOMYPC, etc. Any of these programs will use one computers Desktop.
What Operating Systems Can I Use?