What I would give to be riding in the 60's on a regular basis right now....

Seriously, All you will ever need is a thin windbreaker and some knee warmers (or knickers). I usually cover my knees below 60 as the old joints get stiff. You might also consider adding to your collection a thin set of arm warmers, toe covers for your shoes, thin full finger gloves, and a very thin headband. You'll have all the options you need for those temp ranges.

The best thing you can do to acclimatize is ride in the cold as much as you can, your tolerance threshold improves.

To your OP question - about -20F is cold enough to keep me off the bike.