Guitar strums are quite hard to program so there are various midi file templates for guitar strumming etc I know Twiddly Bits do one (sorry don’t know the web address), also there are a couple of auto accompaniment programs that could be just what you are looking for Band In A Box would be ideal for you I think the web site is at
www.pgmusic.com and you can download a demo from there or there is a program called Jammer Pro which I believe is quite good but again don’t know the web site, but any search engine should pick it up.
Good luck.
DK