Hi DM,
I can give you some help on this but the answer is far more complex than I can explain on a single post. So I'll try to make it simple and hope that other users help so you can understand along the way. To start with I can say that your “plan” is fully executable – but probably not in the way you're thinking. Although the PC platform to produce music is becoming more and more popular as opposed to the “hardware” perspective I personally believe that for now the good way is some where in between. If the PC is more flexible you've got to consider to have the fastest processor, the highest RAM possible, a large hard-disc, a good audio/midi interface (you have to analise your needs on this namely on the number of audio inputs/outputs) and of course the software. The other side on this is that, has it happens on music “workstations”, you've got to continuously update/upgrade your system (I wont bother you talking about the PC/MAC& Windows/OSX saga). So there it goes the financial advantage. That's why I personally think that the best is to have a mixture of both worlds (if I can say that). Now you have you're PC system ready and you choose a software workstation like Reason, Sonar, Cubase etc (please make a search on this). What you can do now is buy not Patches but instrument plug-ins (some are included with the original programs). There are far to many to name them (you have also to search for these) but what these generically do is to play a specific (or generic) kind of instruments/tones/patches. So you can have one plug-in to play orchestral sounds on one track, another to play organ in another track and another to play drums on a 3rd track. Or you can use the same plug-in on several tracks to play different instruments. What the plug-in can play or do depends on it. Some are specialized on piano, some on virtual analog synthesizers, some on orchestral and so on. There are also plug-ins for processing sound like reverbs and compressors. After choosing you can buy the “patches/waves” to suite your needs or load them with your own. You can easily download demos from the net to have an idea of all this and try for your self.
Hope to have helped you to start in some manner.
Regards