I agree, it's not hard to ride a 17in rim bike off road at all. Yes, at 19 or 21 will be better at it but the smaller rim will not be a big deal unless you ride very rocky or sandy trails. I have a motard and ride that off road with friends on dirt bikes. I can go where ever they go without much hassle.
If your plan is to ride dirt roads and some open trails or fire roads, the hypermotard will do great. It's a very pretty bike though so that might hinder the type of off road riding as you won't want to bust it up. If you plan to do much of any highway riding, these bikes might not be for you. Some food for thought, check out the BMW f800gs or triumph tiger 800 as these will fill your needs just fine. A cheaper solution would be a dr650, although this would be best on dirt but capable of longer rides.
04 FZ1, 06 DRZ400 SM