I'm going to be just a little brutal (i.e. honest with my opinion), but if you've paid a premium to own a Surface Pro 3 (there's plenty of other cheaper laptops out there with touch screens), then you can most probably afford an Office 365 subscription. Yes, it locks you into effectively renting software, but there's a lot of sense in using that ownership model if you think about the cost of buying the software and the added cost of upgrading to the next version when that comes out. As mentioned before, you get a huge amount of online storage, and the ability for your copy of MS Office to follow you around (you can stream the software from the internet on any machine that doesn't already have Office installed - with a decent internet connection, you can be editing documents in Word in under 5 minutes).
You can have this for about the same cost as a cup of coffee a week, and there's a wide range of plans so you can get something that suits you, or you and the family.
Cloud based solutions like this are just plain freaking awesome.