Assuming you're doing mostly BG music you don't even use most of the instruments in a GM set. I now use RTs exclusively however when I was using MIDI I used a soundfont player. You can search the net for free single instruments and then using Soundfont librarian put together a custom bank AND if you download the free Vienna Soundfont Studio you can make your own fonts just the way you like them to sound. You obviously have a guitar and I'm sure you have acess to a mandolin. You don't even have to know how to play as you're just going to record single notes and do some tweaking. I did this many moons ago for bass & the famous DX7 sound and some other instruments. Well worth doing.

John
ESI Gigaport HD+
Lenovo Turion II /4 Gig Ram/ Win7x64 be
15.6" Monitor
"The only Band is a Real Band"
www.wintertexaninfo.com/BANDS/JohnnyD.php