How do I choose a water-based primer?
If you want good paint performance, it's less about the paint and more about the surface preparation.  You have to remove dirts and oils from the old surface, and scuff up the surface so there is some microscopic roughness for the new paint to adhere to.  

Scuff-sand with a Scotchbrite style kitchen sponge lightly passed.  If you're galling up clumps of paint you are doing it too hard. There's no significant risk of lead dust, but if you're worried about that, keep the sponge wet instead of moist.  

I hear "can't sand".  The paint doesn't care.  If you don't roughen the surface, it won't stick and will fall off in a few years.  

Clean with Tri Sodium Phosphate or equivalent product, but thoroughly rinse it off. 

If you're looking for a "product in a can" that will do this prep for you, there's no such thing. 

As far as thickness, emulsion/latex paints are supposed to be very thick.  Most come out of the can ready for application by roller, brush or airless sprayer.  If you must reduce, follow the can's instructions. 

If you have a very challenging surface, you need first tip-top prep, and if you must hire it, do so.  And then you need an oil primer. Fooling around with water based primers is a risk.   



Once the prep is done, if I must use a water based primer, it's Kilz 2 simply because it's available everywhere.  For bare metal I insist on at least oil based Rustoleum 7769 Rusty Metal Primer (which still needs prep to SSPC-SP2 bare minimum, pref. SSPC-SP3).