Well, the Garritan Jazz & Big Band 3 gives you many sounds, not only many horns (saxes, clarinets, trumpets, trombones) but also some basses, drums, keyboards. That's a good package for the price. I think it will be ok if you use the instruments as sections, e.g. several horns at the same time. The Garritan runs through the Aria player. To my (inexperienced) eye, there seem to be not as many possibilities to tweak Midi/channels/input/output as with e.g. Native Instruments Kontakt. But maybe I'm wrong. And you can always tweak using other tools in Reaper ;-)
If you are looking more for solo instruments I would rather recommend Native Instruments Session Horns Pro (
https://www.native-instruments.com/d...ession-horns/) or even SWAM engine products (
https://audiomodeling.com/swam-engine/). The latter products are rather expensive. However, many people think the SWAM instruments come closest to the real thing.
I own the Garritan and NI Session horns pro. I'm using mainly the NI Session horns to produce better sounding MIDI for some sax quartets and quintets when I want to play along.
Triggering with a keyboard shouldn't be a problem with Garritan. I cannot say how it would work with your MIDI guitar.
Regards. Jordu