A useful technique for focusing when timing might be critical, or, in the case of the Rollei 35 (which has no way to see how you are focusing) is by pre-setting your lens using this handy Hyperfocal Distance Calculator.
Finding batteries to replace the old mercury types aren't so easy, but zinc air varieties are available. Although once they have been "opened" they don't last more than about 6 months - whether being used or not- they are an environmentally (and legal) way to go.
An excellent resource for collectable and hard to find cameras is Stephen Gandy's CameraQuest.
For calibrating your monitor for photo viewing, I have not found a more comprehensive set of scales than the National Archives Monitor Adjustment Target.