How to prepare & paint a porch that has existing lead paint?
You need to remove flaking and loose paint or else it will eventually chip off and be a hazard in itself.

One approach is to avoid sanding and do a moderate scraping with a carbide paint scraper.  This will generate some flakes which can be much more easily contained than dust from sanding.   Vacuum with a shop vac and wear a mask.

Then prime and paint with a good outdoor deck paint.  The surface will not be perfect, but is should be good enough and the risk of freed lead paint is minimized. Besides, 100 year old houses should not have glass smooth wood surfaces! (my 144 year old one doesn't).