Hi John,
Here are my thoughts:
1-Forget about General MIDI (GM) sound sources. The best sounds come from non-GM synths. I use Kontakt as there are a number of good third party sounds for it as well as NI's Kontakt sounds. I use the full version of Kontakt but you may get buy with the free Player version. There are many sounds available for the free version but you must read the requirements carefully as some will only run on the full version.
2-Your options are not limited to soft synths like Kontakt. Many hard synths (keyboards) have excellent sounds also.
3-You will need to learn about the instrument you are emulating and how to get those nuances by using MIDI controllers (CCs). Don't let that scare you. One it is not that hard to learn and two many soft synths already have the nuances programmed in and ready to use via key switches, knobs, or sliders. Most are assignable to hardware controls, which leads me to:
4-Buy a MIDI keyboard controller, especially if you are using soft synths. It doesn't have to be an expensive one but I would strongly suggest you get one with pitch bend, a mod wheel, 8 sliders, and 8 knobs. The sliders, knobs, and mod wheel are assignable so you can match soft synth controls to the hardware MIDI keyboard controller controls. It's super easy to do now-a-days.
5-You are not limited if you don't have a MIDI keyboard controller. You can manually input everything but that is tedious, time consuming, and I find not as realistic if done while playing with sliders and knobs.
6-I would strongly suggest you do this in a DAW and not in BiaB. A DAW has many options including sound on sound recording. Many times I will record the notes of a song then using sound on sound recording adding the nuances via sliders and knobs. DAWs have their own terminology for sound on sound but all of the good ones have it.
I hope this helps and if I can help feel free to ask.
Good luck.