Hi - just got back late Friday from my M2 ED trip.
ED or not, an M2 takes about two weeks to build. They recommend ordering three months ahead of time, but it can be done in less time; however, they don't build a lot of M2s, so you should get on a waiting list with a dealer right away. For an April 2018 ED, you'll need a first quarter 2018 allocation, & those will be released to dealers about two months from now in mid-Dec.
Dealers don't automatically give the 5% ED discount - you likely have to ask for it, but you should not have to fight for it. Some dealers are refusing to honor it, which is their prerogative, but if that is the case, keep looking for another dealer who will. You're not likely to get a further discount beyond that, however.
From my experience, you can fit four adults in the car, so long as the back seat passengers are no more than 5'9" tall, otherwise they will run out of headroom; however, the trunk is not likely to hold the luggage for more than 3 people (and that is based on small or medium size suitcases, NOT any large ones, and one backpack or similar sized carry-on per person).
Czech and Poland are both on the list of countries you can drive the ED car in.
I would not be overly concerned with car theft - just lock the car and NEVER leave the registration & insurance papers in an unattended car while you're on your ED trip (they will warn you repeatedly against leaving the papers in the car).
Yes, in most cases, the dealer will take a deposit at the time the order is placed. And, as you mentioned, it has to be paid in full 7-10 days before delivery.
I would not suggest you look forward to any hotel or flight deals - the ED packet that BMW sends out after you complete your ED Purchase Order has some alleged hotel deals, but the hotel cards they send are hopelessly outdated and the offered prices are quite high. As for flights, BMW used to offer discounted tickets on Lufthansa but that disappeared years ago.
Best of luck - it's an incredible car and an incredible experience - both met or exceeded my high expectations and I would not have done it any other way - for me, it had to be ED & I had to at least have the 5% discount.
