Is motor size that important?

I am 5'8" 175lbs is a 3hp motor enough, or am I really going to regret not getting 3.5hp or bigger? I am training for a marathon, but currently running mostly outside. I plan to run outside as much as I can, but I also live in Minnesota, so snow will be falling in the next few months and trails/sidewalks in my area are not kept clear, so I will have to run indoors. Another member of my household that is about 275lbs will also use the treadmill, but only for walking and for about 30 minutes at a time.