Depends on how long you plan on keeping the car.
If your going to sell it in the next year, and you will do low miledge, then it MAY be worth it, I dont really think so, but it may. I say this for the reason below
E90's are now old, F30's are face lifted, and the G series's will hit our streets in about 2 years, thus that means that E90's are now really old, and are worth nothing.
If you plan on keeping the car, and doing another 100,000km on it, and I dont see why not then Dont extend plan.
Take that 5 or 6k you were going to spend on the plan, invest in a new water pump, radiator hoses, good service, all new belts, tentioners ect.
This is after plan is up, see how much of that ou can do in plan.
While in plan, take it in, complain about noisy suspension, clunking/rattling, en get some control arms out the deal.
If its an auto, complain about sticky shifts when cold, and see if you can maybe get a gearbox oil change out of them, but now your pushing it, alternatively have ZF or I believe OBDWorx is heading that way.
Think of it this way, whats the worst that can happen, you pop a motor. Replacement from Japan auto is like 20k to 25k for a complete motor. Thats a lot of money, I know, but that is about the worst that can happen,
A gearbox second hand in auto will cost you about 12k, if you have it serviced properly now, or just after plan, they are good for 200k
Turbo's could be an issue, those are about 8.
But BMW's do NOT explode on 101,000km on the clock, contrary to popular belief.
Depends what you use the car for, and how you drive..
More questions then answers huh.
But its an E90 320d... come on. maintenance is piss easy, and finding guys to work on it is even easier.