Wyndham, there are three free expansion packs for DM Pro. You can get them here: http://www.cakewalk.com/Support/Knowledge-Base/2007013208/Free-Dimension-Pro-Expansion-Packs-1-3

You may not need anything else right now. I would use a DAW, if you don't have a favorite one use RealBand (RB), it is a very good DAW, I say this because in a DAW you can have multiple VSTis playing at the same time. For instance say on track one you have DM Pro playing bass and on track two you have another DM Pro playing sax. I believe you can have up to 48 different tracks in RB and that is more than enough to use. So technically if your computer can handle it you can have 48 versions of DM Pro playing at the same time.

I hope this helps.

PS - as you are finding out very good MIDI sound sources cost money! If you are going to get one really good VSTi then definitely get Kontakt.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware