Well, if you go with midis' you can always try to find someone to patch them and use high quality patches and convert them to mp3. Kinda like what I did with a sky chase midi. I had it use a nice sounding sax and it really improved the sound...
Problem is, it's tricky to find good sounding guitar patches, unless you buy/get one of East West Quantum Leap's stuff... http://www.eastwestsamples.com/details.php?cd_index=1032
or http://www.eastwestsamples.com/details.php?cd_index=1059
Yes, those tracks pretty much were converted from midi... Problem is, it's hard to find good free stuff...
Midis could work... at least starting with XP midis are played using MS synthesizer and sound the same on all PCs...
Your other alternative is looking for some creative commons music like from here http://jamendo.com'>
http://jamendo.com where you can pretty much use it for whatever you want, based on the license... Most of them is just use it but give credit...