General Cycling Discussion - Alternate rim/tire for occasional use?

I have a Trek 7500 with a 28 spoke rim/700x35 tire that has limited tread pattern but is targeted for street use. It works fine for 95% of what I do (ride for excercise and pleasure about 70-90 miles per week). However, I want to take occasional short trips (100+ miles with a light load) that are on unpaved trails. A typically trail is hardpack crushed rock like is on Katy trail in MO. I am concerned that my 700x35 street tires would give me problems and I am thinking that a wider and more aggressive tread in something like a 700x45 would be more suitable for that type of trail. If so, I would like to get a seperate set of rim/tire's that I could change to for that type of riding. Durability and handleing is my main goal but I don't want to sink a lot of money into these rim/tires.
I am looking for opinions of others who have "been there...done that" to tell me if I am on the right track and any suggestions on an inexpensive approach to doing this.

I would not worry about something like the Katy trail, I have seen pix of it and it is flat and smooth. Using 700x38 tires with moderate tread I have done dry rocky single track with a heavy load in my panniers (270 lb total load). I would not worry about the Katy with 700x32 tires. Save the balloon tires for mud and tree roots.

I live within 2 miles of the Katy Trail so I ride it fairly frequently. 700 x 35 tires are plenty wide enough for the Katy Trail. The tread pattern doesn't matter either.

If you decide to get yourself a spare wheelset, make sure that your rims are exactly the same width. Otherwise you'll drive yourself crazy adjusting your brakes in and out.