4-5 days of rentals is enough to pretty much buy a pair of skis that will last years and you can use whenever you want. Not to mention pow skis are so good these days for most people there's no new skis that are going to revolutionize their pow skis experience...
I think you have two options. A: Buy something ~110 underfoot that will do everything well- something like the Praxis (great compnay) MVP or Line Sir Francis bacons. Both would do well for some park laps- but kill it everywhere else on the mountain.
Or you could get something 90-100mm underfoot for park and low snow days, and then get something fat 120-130mm underfoot for pow days. 6in of snow is a lot more fun on a big fatty than a normal ski- so they aren't just for when it really dumps. If budget is tight, look for some used skis on the forums and ebay. I picked up a pair of older K2 pontoons for under $300 with some jesters, and they were in fantastic shape (a lot of big fattys don't get a lot of use because of their inherit intended use, so you can get lightly used ones for a big discount simply because they are a few seasons old). Sure my pontoons are a 7 year old model- but they are a freaking powder ski. Tons of surface area, a lot of rocker, insane float- what more do you need? Soooo much fun on the pow days we had this past season, one of the best ski purchases I have ever made.