Hi David,
There are many ways to set up a soft arranger. The commercial venues offered to you in this thread are among them, and although I haven't tried either, they appear to me that they would be quite viable, but perhaps also a bit costly for an initial setup.
Having done this for a little while myself, I recommend starting out with what you already have ( a computer and perhaps a midi capable keyboard ) and low cost modules ( along with some very decent free ones ) .
There is a thread on the software arranger forum in which the developer of One Man Band readily admits that hardware might work better for most users. While I appreciate his candor, and actually agree in some ways, at this point I like my software system better than any hardware arranger I've owned. Not for convenience sake, since I do find it easier to navigate the more complex functions of an arranger on a self contained hardware surface. The one reason I like it better is the sounds. Unmatched by anything hardware I've owned to date, but I haven't owned any of the latest either ( Tyros 2 G70 etc ). My two current hw arrangers are the PA80 and MZ2000.
OTOH, I use my Motif ES as my controller, and it has enough assignable knobs and controllers that I can play live very comfortably with it. I don't have them labeled as a real arranger does, but with jiust a few hours of practice, my hands and feet learned exactly what to do in order to change variations and do fills, etc. Kind of the equivalent of learning to drive... It's not something I have to consciously think about any more.
I won't tell you it's easy to setup and get running so that it becomes your unique instrument ( it isn't ). It's time consuming, with a pretty big learning curve. It can also become costly if you start adding the highest quality sample sets and synth modules. Is it worth it ? Definitely yes, for me. Maybe or maybe not for others.
Chief adavantages for me :
Ability to customize my soundset and supporting setup exactly as I like it.
Better sounds than any hw can get, and you can always add new ones at a fraction of the cost of a new kb.
Chief disadvantages for me:
Not as user friendly in a live setup as using an "all in one" HW arranger.
Spent a lot of money, sometimes on things that I get and later don't find to be particularly useful inside of my setup.
There is plenty more info here at SZ. You might try a search for FLR ( Frank Rosenthal's name for his awesome system ). Also Rikki has quite a lot of experience with this stuff as well.
Best Wishes,
AJ
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AJ