first of, put 10k down and finance the rest at less than 3% interest. Keep 30k in the bank for other things (intercoolers, tunes, etc...)
Low 40s should be a pretty decent price for an IS of that vintage. CPO, may be several grand more, but well worth warranty. Be flexible on options and colors, 335is used dont last very long. Once you want CPO choices are even sparser, and you may need to travel to get yours.
I was lucky to find CPO within 300 mile radius, and bought it site unseen. Broght down payment, signed the papers, and drove off.. And what a drive that was on the way home..
Settled a little on options, but did get the color I wanted.. No Nav, or convinence pack, but every thing else. Even though I like the way 19 inch rims look, 18s are much more comfortable for daily drive..
Good luck, awesome cars.. I am extremely happy with mine, and have 80k warranty remaining with extended maintenance plan as well.. I got smokin deal on it!!