finalising a postdoc employment contract
Having done a post-doc in The Netherlands, I think I can answer your questions.


Your "yes" is an acceptance, though probably not legally binding. They will take it in good faith that you have accepted. The contract may arrive in the mail for you to sign, but you may just sign on your first day. 
The starting date is the date you will start and get paid from. You will probably need to go to The Netherlands earlier to find accommodation, though you could easily start everything on February 1 without causing any problems. They will understand that in your first few weeks you'll need to vanish to register yourself with the city, to get bank accounts, etc etc. 


I would ask them if the have an accommodation service, even if it only provides short term accommodation. Finding accommodation in The Netherlands can be difficult. 
In fact, I would simply ask them what their expectations are, essentially sending them the questions you've posted above. Unless it is the first time they've hired a post-doc, these questions will be unsurprising and they should have answers for them. The Dutch are good at dealing with direct questions (and you'll need to get used to receiving them ;-) ).