I honestly don't know about the upright question... I built my own mill. HUZTL has a 42" mill that will hold from an 18" bar up to a 42" bar for like 150 delivered. I have never upgraded the bearings in any of the kits I have built (over a hundred) over the past few years, and have never heard from anyone about them failing. I have sold many saws country wide to many logging companies. They all have my personal number and I remain in touch with them... I can't personally comment on the bearings because I own my saws for a short period of time... I build them, then use them then sell them. It's been a fun hobby basically. But I would say the failure rate is low based on that knowledge... Now, that being said I am not a daily chainsaw operator, and am not using that information to persuade you one way or another. There is a lot of different opinions on many components within these kits... Bearings included. My personal feeling is such that I build them as a hobby and they are easy and cheap to fix so downtime to me doesn't correspond with money loss. If mine breaks, I just tear it apart and put it back together so inexpensive ones don't scare me. Most saw operators don't want the risk... I would urge you to do actual research into it and make your own personal decision on what parts to swap out.
If you are planning on going to use no more than a 36, why go with the 090?? Again not trying to persuade you either direction, just curious. The ms660 is rated to do a 36 and has plenty of power... If you throw the big bore on there you have all of the saw you will ever need and 10 lbs lighter?? I get it though, if you just simply "want" a 090 and the price is right. I want one too, but I never want to actually run it because of it's size!!