My $0.02:
1) If you can only go Saturday, get there early so you can go right in when the doors open. The halls will not be crowded, but the meet ends at 2 p.m. and people do start packing (to various degrees) around noon. Plus, they have free coffee and a doughnut before the meet opens on Saturday in the entry area for Orange Hall.
2) Finding PRR stuff will be no problem. You will run out of money long before you run out of PRR stuff to buy.
4) Keep an eye on the York section of the forum - sometimes a thread gets started to list locations of table-holders (I'm usually in Blue Hall). You will run into some people at the OGR booth in Orange Hall, but the best place to meet fellow forumites is the OGR gathering at Quaker Steak & Lube on Thursday night (if you were going Thursday).
5) There are usually 1-3 club layouts, and they can be in Orange, Black, and White halls, depending on a number of factors. Announcements on how many clubs and where they will be set up are made closer to the meet. Also, MTH, Lionel and Atlas have small demonstration layouts in Orange Hall.
6) If you are looking for modern production, I would start with Orange, Brown and Purple Halls (the dealer halls). Used modern stuff can also be found in the member halls (Red, White, Blue, Silver) at sometimes very good prices. If you are looking for postwar, the member halls are the place to be.
7) There are dealer hall charts available on the Eastern Division's web site. They do not change much from meet to meet (all the big vendors are in the same locations), so you can look at the ones from the Fall meet until the updated charts are available.
There is a lot of useful information on the web site above (7), so I suggest spending some time there so you can formulate a good plan of attack. And I highly recommend bringing a copy of the fairgrounds map with you. I forgot it on my second trip to York and I almost forgot to hit the White Hall (and a great bargain I found there).
Andy