When you purchased the 335 instead of a 328, what was your reasoning? Performance?
If you're not looking to use the 135 for track days or anything, then I think you'd be plenty happy with the power of the N52. It's definitely not as quick, but as mentioned...it's not slow either; just depends what your measuring stick looks like.
Analyzing your post, you really seem like you want the 128 due to price (you're gut told you $50K+ for the 135 didn't feel right, add insurance cost difference to that too) and because you appreciate and find joy in the NA engine over BMWs new direction with turbos.
Is there any action in regards to BMW meets in your area? Cars and Coffee or the like? If so, maybe partake in those some and hope someone can offer a 128 or 135 for you to get your hands on, or know someone who does.
Both will do fine with a good quality set of winters, it's not a XI by any means though. You're only struggle will if you live where the roads arn't plowed often enough as there is a lack of XI ground clearance in the deep stuff. I lived in the lake effect snow belt near Cleveland, OH and as a native Texan...drove just fine through 5 winters with RWD only BMWs (the ex's 328XI with A/S had problems, albeit rarely, when I wasn't struggling for grip).