I've been doing this for my gaming group for years but have only been kicking the idea of building my own adventures for resale during the last couple years. The tricky part for me is setting the encounters up to be challenging but not overwhelming.
Hehe, reminds me of a story - We don't play often but I have 2 adventures going in the same world for the same group because whenever anyone can't make it we run a little Mercenary company for whoever is there. The last time I set up an encounter for this I may have overpowered it just a little as the full party wipe bears witness.
MUAHAHAHA - FEAR YOUR DM!
I wish you success with this and I am interested to hear how you find the process to be. I would be willing to help out but I think my lack of D&D stock monster knowledge might be a weakness.
There are some nice references out on the web for 3.5 stuff. I found a pretty slick site with ratings to show difficulty as well as descriptions and illustrations - pretty much a Monster Manual.
Personally, I kind of like the more generic adventures where they have a story line and allow the Game Master to load up the encounters with appropriate stuff. I think that's better than saying, "There will be 5 bugbears here ready for action". I think the GM should be able to know if 5 or 10 is a better challenge for his group. But then, maybe that's just me, it could be lots of folks want it all spelled out for them.