How should I prepare a concrete slab for plank porcelain tiles?
You should try to get things that have height off your floor.  A little glue stained on your floor, paint, stuff like that - won't hurt anything.  The thinset will bind to that just as good as the concrete.   So you want to make sure anything sticking up is scraped and anything that is loose.  

Leveling is not a must.  Most basements slant towards the drain.  Whether you level your basement is based on:


How big the tiles are.  The bigger the tiles the more level the basement needs to be. If you were doing mosaics you could handle any slant.
Whether you care about drainage.  You may want to keep the slant on a basement predisposed to have water issues.
How good of a tiler you are.  If you don't have experience tiling, a floor far from flat can be really challenging.  You basically have to use different amounts of thinset to even out your tiles.  I have seen this turn into a disaster for some.  Definitely takes some planning.