replacing galvanized + lead pipe???

Hi,

I Am redong my bathroom and am down to the studs. I found galvanized pipes for the sink drain,vent, and the sink and bathtub drain in bathroom directly above. I even found a lead drain pipe that runs horizontal for about 2 foot then connects to the vertical cast iron pipe that runs straight down the wall into the basement, this lead pipe is for the bathroom toilet above the bathroom I am redoing (so it is above the ceiling I just ripped out.

My question is this: nothing leaks at all currently, so I am not sure if I should maybe replace everything, or maybe just the lead drain pipe, or the galvanized drains?? I am on a tight budget so money is definitely an issue here. These pipes are 55 years old and look to be in decent shape.

One more question.... The galvanized vents only vent gases and air so these are prolly much less likely to fail b/c they don't hosue liquids correct?

Any help on what I Should or should not replace would be extremely helpful!!

thanks in advance,

Harvey