I did this project tonight, only I made a swage block instead of an anvil. Here's what I came up with.
As far as yield goes, I ended up with a 15.5" x 10" x 3" swage using one 4' x 4' x 1/4" mild steel plate. You'll get more out of it using 3/16", of course, but 1/4" is what I had handy. It comes out to something just under 125 pounds (the swage has cutouts, so it's not a full solid block)
Going off the advice of our welder, who is very good at what he does, I cut the plates with 1/2" half-round cutouts every inch along the perimeter, and 1/2" plug holes which were offset from plate to plate. In the case of the swage block, I also cut gradually smaller circles in each plate to get some rough dish shapes which I'm going to grind smooth tomorrow. I'm not sure how your plasma cutter compares to my laser, (It's probably faster, actually) but it took about 45 minutes to run the sheet.
Since he was busy with some other stuff, I did the welding on it (and I am not a masterful welder in any sense of the word). It took about 2 hours, with occasional stops to change sheets on the laser on the other end of the shop. I lined up each plate and clamped them down, then filled the plug holes and welded the inside edges of my dished cutouts and ground the top of the plate smooth before moving on the the next one (be careful with this- I started to get a little rushed as the shift was drawing to a close, and it's very easy to get some small gaps- I have two on one side that are about .010" wide, probably from weld spatter I missed. Make sure you have a few clamps, as the plates will start to warp a little from the heat, especially when you have got a good base built up and it's steaming hot.
After that, I welded up the cutouts on the side, and filled them a little more than flush with the surface. It takes about three passes. When the sucker was done, it was HOT. I had another guy help me carry it out to the parking lot with a couple of giant C-clamps, and let it sit under the garden hose for about 20 minutes before it was cool to the touch, and I suspect that the core is still pretty warm even now- it steamed for at least 10 minutes, even with a good flow of cold water running over it continously, so plan accordingly- especially if it's gov't work, and you don't want to leave it sitting around to cool.
All in all, it was not that hard to do with a wire-feed welder (though I still have the grinding to do, so I may change my tune yet,) As I was working on it and thinking about the project overall, it occured to me that if a guy was making an anvil, I had been thinking about it the wrong way- instead of building it up in layers like a cake, from the base to the top, I think you'd be a lot better off building it from vertical plates cut to the anvil's profile- that way, you'd be avoiding a lot of the trouble that gaps could cause. A good layer of hardfacing rod on the top would hopefully keep it from delaminating, and cover any gaps that might be there.
I know that still doesn't fill in any gaps about how well or how long this thing is going to hold up, but I figure that a brief description of the process might be handy for you. After I use it a bit, I'll let the group know if it was a worthwhile project, or if it just breaks apart once I start hammering on it. I got very good weld penetration (I know what to look for, I'm just not that good at welding pretty), so hopefully it's enough.
For the price, (free) it can't hardly be bad- though it's not likely to be as good as one that is cast in a professional foundry.
If I were doing it again, I think I may have tried sprinkling borax between the plates and trying to give it a quick and dirty forge weld by heating each plate with the oxy-acetaline torch as I built it up and then welded it with the wirefeed. I have no idea if that would have worked, but it's worth considering- it certainly couldn't hurt anything. Even a half-assed partial forge weld would be better than none at all, if the bulk of your anvil's strength is coming from an electric welder.
Also- you'll almost certainly forget something important if you just wing it with the design. I'm already regretting that I forgot to put in a hardie hole or two in the block, as it's going to be a lot of work to do it now, if I even do it at all. Wouldn't be a bad idea to find some dimensions and copy an existing anvil before you start.
But for the investment in time, it's a hell of a deal. Swages and anvils are more or less similar in price, and I figure that for what I ended up with, and even if I would have had to pay for the full cost of the materials, I basically paid myself about $100 an hour to do the project, and that's not half bad for a shop rat.