Hi "$00", it's never to early to make reservations at York. You may find that most of the rooms are already taken up by the dealers and regulars.
As for the best town to stay in that is a good question. If all you wont to see is York then that is the best place to stay. But you did mention that your wife wonted to see a few museums. In that case I would look to stay in Lancaster, PA. on Route 30 near the outlet centers.
From there you are only about a fourty-five minute drive to York going west on Rt 30. The RR Museum of PA is only about a couple of miles away, along with the Strausburg RR and the Toy Train Museum.
Another option, though a little more expensive is the Baltimore Inner Harbor. It too has alot of shops and reastaurants but it also sports the Constellation sailing war ship and the National Aquarium. The B&O RR museum is only a mile or two way. I think the drive time from Baltimore Inner Harbor to York is about 3/4's to one hour.
As for the best York to go to I would say that would be the fall show. This is only my humble opinion because others will say differently, but I always felt that the fall screams toy trains. I go to spring meet as well and I enjoy it but the fall is my favorite. I think it has to do with when I was kid, the fall was always the time of year when we would ride our bikes to the stores to see what was new from Lionel.
Well I hope I was of some help to you. hope to see at York.