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Based on my experience with a 2008 335i that spent its first 6 or so years in hot Houston, TX and the subsequent 4+ years not too much farther north, with ~100,000 miles at time of hose connector failures/replacements, I would replace the following:
1) As mentioned above the hose with the plastic fitting that connects to the front of the head, had the plastic fitting shear completely in two and dump all of my coolant when I was at work. The hose was still in good shape and the opposite end was a pain to get to, so I replaced the plastic fitting with the aluminum ebay fitting mentioned above.
2) After the above failure, the upper radiator hose was being replaced proactively before failure and the small plastic nipple near the radiator that supplies the vent line to the reservoir broke off while attempting to remove the hose. This was a failure waiting to happen.
3) Upper vent hose that runs from the upper radiator hose to the reservoir has a rigid middle section under the air box supply duct that is clipped in above the radiator. When I attempted to remove this hose, the rigid middle section crumbled at the retaining clips. Another failure just waiting to happen.
4) I also replaced the lower radiator hose proactively due to the aging plastic end fittings, but honestly, that hose and fittings were still in good shape. I suspect it sees much lower temperatures than the previously mentioned hoses because it is on the cooler outlet side of the radiator.
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