While Demoing is good it is not the be all and end all of buying a ski. Threads like this seem to always end up with the regular chorus of demo demo demo. I disagree for the following reasons:
1. Often times demos have a limited selection based on size and model.
2. That selection often doesn't include some of the best skis (ie. DPS, PMGear and other "indie" companies that churn out awesome skis).
3. You only get to spend between 1 run and a couple of days on the skis. Personally, I think some skis take a little longer to really learn how to ski them and push them to their maximum. I've owned some skis that I wasn't a huge fan of the first time I rode them and I later learned to love them. This might be especially true for you if you haven't skied a lot of true powder skis before. Not all of them are easy to ski the first time.
4. Tune and mount point can play a huge part of how the ski feels underfoot. I have my lotus 120s mounted at -1 and I love the mount there. At the center line they are still good but not as great as at -1 for me atleast. Also a bad tune can ruin your entire image of the ski.
5. Conditions may not allow the ski to shine. If you are trying to demo the bushwacker in 2 feet of fresh you probably wont have a lot of fun, just like if you were trying to demo the pontoon during a dry spell. (this seems obvious to most).
The long and short of it is, yes demoing is good. But its not the be all and end all, especially if you are only taking one trip out west. I have demoed skis and hated them before and it almost seems like that whole day of skiing wasnt worth it. You have to be willing to risk that if you decide to demo a ski for a week on your only trip of the year. You also play this chance when you buy a new ski but there is less daily change and that allows you to adapt and really discover the ski.
My recommendation would be to check out blistergearreview.com. They have some awesome reviews of a lot the major and "indie" ski companies skis. Not to mention their reviews are really top notch.
You can look at a ton of different skis to accomplish what you want. Anywhere between a 105-125 waist could work depending on what kind of sacrifices you want to make. I'd look between 180-190 length for your size and weight. 170 will be way to short as a pure powder ski for someone who is 225lbs.
Personally, I think the 184 or 190 Wailer 112 RP, 182 Moment Jag Shark, 183 or 190 Volkl Katana, 186 Blizzard Gunsmoke or 186 PM Gear Lhasa would all work great. If you are interested in a 190 Wailer 112 or a 186 Lhasa I have some I could sell you. Let me know if you have any other questions.