The ones Enco sells are usually fine. I have a 12X18 I bought from them about 5 years ago. I have seen the Enco plates (and similar) used in scraping classes and they all proved to be of sufficient accuracy for the work we were doing.
You do not state what class of work you are doing. There are different classes of surface plates. "Shop floor grade" (Enco grade B "tool room") is fine for most everything a home shop machinist/hobbyist needs to do. The higher grades "lab grade" (Enco grade A "inspection") surface plates cost more for the same size and if you are doing the class of work that requires traceable documentation then you will need to certified by a metrology lab.
You mentioned using an edge of the plate to verify the ways on a lathe. Usually surface plates have only one gage surface, the top. The sides may or may not be square to the top and flat. There are much handier tools for checking lathe ways. There are granite and cast iron straight edges with 1, 2, or 4 gage surfaces. Ones with 2-4 gage surfaces are usually supplied in pairs and called granite parallels. Enco does not sell granite straight edges and parallels AFAIK, but Shars does. An 18X12 granite plate would be a very difficult way to check the ways of a lathe.
You also mentioned something about rubbing the ways of the lathe using a plate. Unless I am reading what you said incorrectly that would be a very bad idea for the plate and the ways. WHen you check the ways of a lathe they should be spotlessly clean and NO burrs. A very thin layer of blue is applied evenly to the straight edge and gently placed on the ways. Then the straight edge is shifted slightly once or twice, about a half inch or so. Then the straight edge is carefully lifted from the ways and set aside (blued side up.) Now you can "read" the blue markings left by the straight edge on the ways and it will tell you where to scrape next. There is a LOT more to it than that. Best thing to is to take one of the classes organized via PM with Forrest Addy or one of the other machine fitting experts.
While the tooling suppliers like Enco and Shars sell very inexpensive plates, straight edges and parallels be sure to check the shipping costs because that can add to the price considerably. If there is a place that sells new or used plates near you (within reasonable driving distance) you may be better off buying from a place where you can pick it up.
-DU-
