I pulled my utility trailer when I was running a 4 speed drive on my rigid mtb and have towed 80-100 pounds up some pretty long steep hills but also spend a fair deal of time climbing those hills on a fixed gear pushing a rather tall gear.

My new tow vehicle will be my RM Blizzard mountain bike as it has a 22:32 low gear and should allow me to spin my way up climbs with some significant loads and has the right brakes (disc / v combo) to provide adequate stopping power.

V brakes / cantis can provide adequate stopping power but if you take to towing a trailer regularly keep an eye on your brake pads to check for wear as this will be greatly accelerated as will the wear on the rims.

One another thing to note is the build on your rear wheel... you want the wheel to be properly tuned up and able to withstand the increased lateral stresses you experience when turning if you are towing larger loads. This is why I like having a 26 inch wheel bike for towing as the wheels are stiffer and stronger.