You need your missus' permission? You should show her who's boss!
In theory the lenses should be cheaper with the Pentax having in-body IS but the reality is that lens prices can be more or less expensive that the Nikon equivalent. IS is helpful but it's not always a must have - if you're using a tripod or shooting at a high shutter speed, you don't really need it.
The 18-200mm gives you a lot of flexibility but you have to bear in mind that the convenience comes at the cost of quality. One of my old workmates has that lens on her D5000 and she likes the convenience but seldom uses it and prefers to take an 18-70mm and 55-250mm.