Hi!
I own a Motif ES8 which has the same keyboard action as the S90 and from my point of view it has the best weighted keyboard action as well as keyboard sounds. I'm a big fan of heavy keys. Before making my choice and purchasing the MOtif I've owned and checked out just about everything else out there and nothing came even close. The stock sounds on the S90 range from keys to synths. If you feel that you have no need for orchestral sounds or pads don't worry. THe S90 has a lot of everything and by having extra stuff that you might potentially find useful you're not going to lose anything. It's not like you're going to have a hundred thousand pads and strings and only two pianos. It's all there in large amounts. Besides, you're not paying a fortune for all those extra sounds that you think you're not going to need. (yeah right! You'll use it all eventually, trust me).

Now, you're totally right about the pain of trying to do organ leads on a weighted action keys.... That can hurt! Or mess up the keys themselves....
Getting an additional 61 note light action keyboard is a good idea indeed. To save money you will probably have to get a hold of some older controllers or synths.
I personally use really old stuff like the DX7 (best aftertouch and keyboard feel - period. You will have to get an updated one though to make it more useful as a controller), Korg O1/W, an A30 (very light) etc.
The list just goes on. There are probably about a thousand different models that you can pick up cheap, so you're not going to be broke :-).

In overall I think you have a pretty good plan already. If you need any more input on this you may want to post at the General Arranger section here. You'll get more replyes that way.

-ED-