You might want to consider a simple evacuate and recharge with some ultraviolet dye to find the leak with. Drive the vehicle until you notice a decrease in cooling performance and then take it back to the shop where you had the work done and allow them to go over it with the UV light and find the leak. The cost is what it is but it is a lot more difinitive than just throwing parts at it. The crap shoot is you will spend the money you might have spent on a couple of hoses and an E&R on getting just an E&R with UV die and a proper diagnosis. If the hoses you throw at it fixes it you are money ahead, if not you just threw your money away. If the shop misdiagnoses your refrigerant leak you at the very least have a warranty and a leg to stand on. Truthfully you may find that your leak is so darn slow that you won't notice the a/c falling off for 1-2 years. I have a few customers that come in for there every other year E&R just because it makes more financial sense to them than stuffing in an evaporator $$. Every once in a blue moon you will find a/c components leaking on your 850 that are not the evaporator but it is very few and far between.

Mark