Pulling analog DC output from a laptop
The 0..5V control voltage needs to drive 50 ohms, meaning that it needs to be capable of supplying 0.1 A. A PC can provide that current from the USB connector but not from the serial port. However, the serial port is easier to programme with than the USB port, or certainly has a shorter learning curve.

You could use an USB-to-LVTTL-serial-port adaptor. This plugs into a PC USB port at one end and gives the 5V supply and the RS232C TxD/RxD signals at LVTTL levels (3.3V). You can get these as a module (look at FTDI) or built into a cable, the latter being less than a fiver.

You can then attach a microcontroller or other IC to receive your RS232-signalled bytes and convert them into an 8-bit parallel byte. This can go to a DAC (may be internal to your microcontroller) and the DAC analogue voltage can go through a power op-amp buffer for the 0.1 A drive strength.

This is one way of doing it and hardly meets your requirement of a few analogue parts. But it does let you make something PC-powered and controlled. You may find microcontroller demo' boards on the Internet that contain some or all of this circuitry for under a ton - have a search around. If you're not microcontroller-minded, this is all somewhat moot.