I generally follow the old truism " Lease a new BMW - Buy a used BMW"
In general I believe it is better to keep your cash, and not put it into a depreciating asset. I would lease - putting as little money into the deal as possible. Although I have never availed myself of MSDs to lower the money factor, many here seem to have used this strategy to their advantage. I also like that a lease establishes a bottom. At the end of the lease, you can buy the car at the price specified in your agreement. Occasionally, it is possible to even buy the car and turn around and sell it for a profit to Carmax. Extra miles beyond 15K only cost .16 per mile - and BMW allows you to purchase additional miles up to 3 months before your lease return date. Moreover, after you lease/purchase your new BMW, you will probably receive an offer for a BMW credit card - take it. BMW credit allows you to redeem points earned, based on dollars spent, to buy additional lease miles. My purchases on my BMW credit card are paying for my "over" mileage on my leases.
Trade car that has excess mileage for new BMW? - It happens, but you should not count on excess mileage being waived. You can always roll your your excess mileage into a new lease. In general, I found the folks at BMWFS to be a good group - better than most - they want you to buy a new BMW.
Check out this concise lease primer - I think you will find it easy and you can run different scenarios;
Alternatively - if you are going to put lots of miles on it - maybe more than 2000 a month, you might want to purchase. I would still go for small down payment - keep my cash - and pay a larger payment each month. Given historically low interest rates - why not use BMW's money. This is great if you find that you love the vehicle and decide you want to keep 6/7+ years. If you trade it in 3 years or you have to go overseas again and are forced to sell - you will take a hit. Either you are going to pay up front with a big down payment or you are going to take the hit when you sell assuming you are upside down. Common denominator - you are going to pay. It is generally true purchasing is cheaper than leasing if you hold on to the car.
Lots of ways to get to the final destination - your X3 35i M Sport. Lease vs. Purchase is such a conundrum - we have our own business so my situation is very different than yours
Hope this has been a little helpful. I can tell you that your X3 35i M Sport is worth a little aggravation - besides your pups will love you for it!