You are planning on doing exactly what I did. I got a 29er hard tail and a set of narrow road tires (narrow for a mtb). I have road tires, mtn tires and studded tires and swap them out. I weighed 278 when I started riding again last year and now weight 218 but I would not have gotten here riding trails. I did a lot of extended aerobic riding with a heart monitor. Local trails are so up and down I'd get anaerobic fast, then get a breather, then get anaerobic again. You may find trails fun but they can't provide (in most cases) the extended workout at a reasonable aerobic rate that most roads can do. I also bought a hard fork because it gets too cold here to use a suspension fork in the Winter. My single bike works very nicely for road and mtn biking. Next Spring, if I am under 210, which I expect to be, I will get a full road bike because I do love riding fast and the drop bars alone give me another mph or so and the close gearing is a nice thing. Till then my bike with the right tires thrown on works great and I think you'll find the same. Buying spare wheels and tires to throw on at a moment's notice would be great but I don't have the money to toss at saving that time; I just change tires as needed for the planned trip. Soon the studs will go on for the Winter.