It can be relatively simple or it can be as complex as you want.

As far as the "free soundfont" issue goes, my opinion is that for me, several make for excellent arranger section or backing instruments, and then you can add quality voices for lead instruments. Good free soundfonts include Magic SF V2, SGM 128 & 180, Real Font 2.1, and there are others.

I think what often gets overlooked here is that your PSR's, Rolands, etc, come preset with effects and everything already set to go "out of the box" Try this.. Turn all of the effects off in your arranger, and see if the styles still sound good. I bet they don't. Well most of the Sf's come "dry" as well, so you see what happens here.

So how do I fix this ? I use OMB, and route it's output to Xlutop Chainer via a virtual midi cable ( Maple ). I open my SF's up in a SF host ( SFZ+ in my case ), and open that up in XL Chainer ( Vst host app ). Chainer also hosts effects that play in real time with the soundfont app. I have quite a few high quality effects, but even some of the freeware effects from Kjaerhaus, Classic, Frazier, and Mda can be rather good. Makes all the difference in the world.

Ax far as lead voices go, I'm a synth nut, so Pro53, Pentagon, Absynth, Korg Legacy, and a few others work well for me. For Guitars, Dasample Electro Acoustic and Music Labs Real Guitar as well as the Sonic Implants Telecaster and PRS offerings ( again the last 2 are soundfonts and benefit nicely from add on effects ). Organs are covered by NI's B-4, while electric basses are handled nicely by Steinbergs VB1 ( freeware btw ).

In contrast, my Motif ES6 is a nice all around workstation, yet I think I can surpass almost any voice on it sonically in the software realm. The softsynths I listed above by far surpass ( for me ) what is available on the ES6. No it isn't cheap, but inexpensive was not the criteria for me. Quality sounds that don't necessarily sound like typical sampled kb sounds was priority one.