I'd agree with harryn on the fixed gantry suggestion if it matches up with the space you have. A lot of the moving gantry machines work well with smaller z axis travels, but it looks like you want to have a z travel about half what your x and y are. This is a work envelope that's scaled more like a milling machine, and that's what your sketches look like as well. If that's the direction you're going, I'd definitely look at the fixed gantry. You might look at the Mori Seiki NV6000 or similar mills for inspiration on frame shape (search on youtube - they show the structure of the machine).
If you're going to be cutting steel and expect any kind of decent cut or material removal rate, you will want a much stiffer machine. Required stiffness to cut material with good performance corresponds roughly to the modulus of elasticity of the material, so while my router has a stiffness between the bed and spindle nose of about 2000 lbf/in, you'd really like more like 3k-12k, for aluminum you'd like something in the 20k-80k lbf/in range and for steel more like 60k-230k lbf/in (depending on exactly what your goals are). That's not to say you can't cut metals at all with more flexible machines. I've cut aluminum and steel on my router, and it works. It's just slow and I get a bad surface finish.
On the noise, if you have a high speed spindle, the cutter will still create some significant noise. When I ran my router in a detached garage, the router running by itself was not really that audible but as soon as it started cutting you could hear it. If you want it to be as quiet as a Bridgeport, you might look into a lower speed spindle although this will limit how well it would work with wood. Small scale versions of this could be using a mini mill head. You can buy these from littlemachineshop.com among other places. I have one with R8 collets that I'm working on putting a DC motor on for doing drilling and milling on my router. Otherwise you could look at enclosing the machine in a noise isolation box.
One more thought - usually you don't want to drill with a high speed spindle meant for wood. You want to limit your plunging to about a 10 degree angle iirc. You can look up data sheets from Columbo to see the kind of usage they recommend for their spindles, for example. So while this can work - you can spiral down into holes, etc., it might not be the best idea if you want to do a lot of milling work where you might need to plunge straight into metal. The milling head, on the other hand, can do this kind of cutting and is why I'm planning on using one as a drilling head.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html