I would like to use latex paint to on old plaster walls in a 1800's house. Some of the walls had to be patched with new plaster where the old plaster was removed down to the lathe. There appears they have been coated with calcimine before being wallpapered I searched the internet on which primer to use before latex and now I'm totally confused. Should I use oil, water based, shellac or Zinser Guardz?
You absolutely cannot beat the shellac primer. If you go this route, get Zinsser BIN. It will cover pretty much anything and stick to pretty much anything. The drawback is the cost, at around US$45/gal it ain't cheap. This stuff uses alcohol as the primary solvent, so you might want to get a respirator that is rated properly to filter ethanol (or maybe not, but don't drive after you use it...). This can be used in most sprayers directly out of the can, it's quite thin.
The oil based (like original Kilz) is good too, and a lot cheaper. It's also harder to apply, very thick and prone to brush marks. And it puts off some powerful fumes (so does Shellac, but that is NOTHING compared to Kilz, which uses some more noxious solvents that will make your eyes water). Oil based primer is going to stick to most existing surfaces. This will not work in some sprayers as it is too thick, even after thinning (assuming you only thin it as much as the manufacturer suggests).
Latex is probably the cheapest option, and also the least flexible. It's a bit easier to apply than Kilz. but not as easy as shellac. You shouldn't need a respirator to use it, though. Latex is only suitable for application over existing Latex. If there is any doubt, don't use latex, there is a chance you'll regret it. I've seen badly applied latex where you could cut a small hole in the paint, and actually slide your hand up and behind the dried paint, lifting a large section right off the wall.
In all cases, make sure you scrub the walls before you do anything. This is extremely important if you are painting after removing wallpaper, because it is necessary to get all of the old paste off the walls. TSP is the best cleaner, but it's not environmentally friendly. The TSP-substitute is almost as good. 80% of the work is in prep; slapping paint on the wall is the easy part.