Steve. A few things.
I really don't think you'll be happy with an electric furnace. Some of the stuff you'll melt can corrode the elements. After the first heat, the elements become brittle. You will have spills. Unless you have a hi dollar controller, you can have a melt down. I would definitely build a gas furnace on the order of the link on jimdocs post:
http://www.backyardmetalcasting.com/hmkit.html
This lid arrangement, though, is too complicated. The easiest is to use a pivot pin, at the back, where the metal lid ring meets the body. Instead of lifting the lid, you pivot it to the side. Very simple. You weld two pieces of pipe, about 3" long. Weld one flush to the top of the body and the other to the lid. Weld about 1/2 of the bottom of the body pipe closed. You put the lid on the body, line up the larger pieces of pipe, and insert the pin. The pin should not be too loose but definitely not snug. For the large #70 furnace, I used 1" to 1 1/4" and 3/4" pipes, I think. Try various sizes for fit. Make the pin long enough so that it sticks up about an inch above the lid. That way, if you want to remove the lid for relining, you simply pull the pin. Don't seal the bottom of the body pipe completely. If you leave a gap, you can clean dirt, etc., out of it. Weld a [ shaped handle, from rebar, to the front of the lid.
Use refractory cement - about $50/bag. Plan on a 2.5" - 3" bottom, sides, and lid thickness for a small to medium furnace. I prefer large diameter pipe for the body and lid - about 1/8" thick - from a scrap yard. Have the pipe cut accurately and grind very flat. Weld a plate on the bottom. Weld 4 short lengths of rebar inside the lid to support the refractory. Use an empty can as a mold for the hole in the lid. Use a soup can for a small furnace and a 1# coffee can for a large furnace. You'll need something round as a mold to cast the walls. Use something you can get out after casting or, something you can burn out. For a small furnace, you can use collapsible stove pipe. For larger furnaces, they make sturdy cardboard tubing of various sizes.
I prefer natural gas over propane because, small propane tanks freeze up, especially in cold weather, and stop working. But, if the furnace is outside, you'll probably have to use propane. Propane is hotter and I've run #20 furnaces on 3/8" copper tubing. You can use an old upright vacuum cleaner as an air supply. I can talk more about the burner later, if you're interested. I need to get up my Adobe Illustrator.
A #4 silicon carbide crucible holds 4# of aluminum. Although that sounds big, it's about right for 50 oz of gold. Go to a foundry supply, on the web, and find the dimensions of the size crucible you want. Plan on a gap of about 2" between the crucible and the walls. This way, you can determine what diameter the furnace should be. A #12 crucible is about the biggest that can be handled with 1 man tongs. Above that, you need 2 man lifters and pourers. I would build a bigger furnace than you think you need. For 9999 gold, you need a small dedicated furnace. If you melt less pure gold, silver, or any other stuff in it, you'll contaminate the very pure gold.
If you want to blow up a furnace, leave it outside in the rain, unsheltered. The one on wheels, in the link, might be a good idea.
I probably haven't give enough info or, explained well enough what I did give, to build a furnace. I need drawings.
Another subject. You mentioned plating out copper and nickel from the sulfuric after evaporating the water from the dilute sulfuric. I question whether this will work but, it depends on how strong you get it. You will have more luck plating out of the dilute sulfuric before evaporating. Use a lead anode.
Also, on how to re-use your diluted sulphuric acid.
