Hey! I'm not going to KU, but I do currently live just a couple blocks from campus, so I may be able to answer some of your questions.
It doesn't get stupid cold - this year was rather mild and it only iced over once or twice. However, with the windchill it will stay below 32 for typically the latter part of December and most of January. You will get to thoroughly enjoy all four seasons.
It's pretty much a mix as far as graduate and undergraduate students. Mass st. bars tend to attract a younger crowd, but I'm subleasing not too far from KU and all my roommates are graduate students. Again, the closer you get to Mass st., the younger and more livelier the occupants are going to tend to be.
I'd honestly check Craigslist for housing. There are a bunch of subleases that are going to be up July 31, and if you grab them in time your monthly rent can be pretty cheap.
On the whole, Lawrence is probably the bluest place in Kansas's very red state, so be prepared to live in a place that is very pro-recycling and pro-equality.