Mike,
My dayjob is plastic part and injection mold design (CNC is my hobby). Many of our customers manufacture molds through us, so we have been involved in all four options at one time or another:
#1. I just finished a mold and am in the middle of making another one. My machine is a lowly converted mill/drill, so your BP should do just fine. If you create a complete CAD model first, the machining will be a large, but straightforward task. If your design is not thorough or if you start machining before the design is complete (believe me, people do it...), the task will be daunting and you will likely be forced to make compromises (i.e. there might not be room for waterlines because of where you placed the ejector pin holes).
#2. I am a strong believer that cheap outsourcing is almost never the way to go. Not just from a keep-it-in-North-America standpoint (I'm in Canada), but from my experience, a cheap outsourced mold is never cheaper than a mold built in NA in the long run. Also, important items are often substandard or missing (such as waterlines, the locating ring, air vents, wearplates on slides, etc.). That means the mold would require repairs before you can even make the first shot. Granted, we have a couple of customers that plan on having to make immediate repairs, so for them, cheap outsourcing works.
#3. By short-run/proto, I assume you mean epoxy molds. Aluminum molds can also be considered proto, but you already mentioned them in #1. Epoxy molds are good for short runs only (because they deteriorate as the parts are made), but 5K to 7K is not a short run. With some types of epoxy molds you might get 1K parts, but you probably don't want to make a new mold every year.
#4. Contact any plastic injection shops that are affiliated with or have their own machine shops. Often you can negotiate a 'free mold' if you agree to purchase a guaranteed quantity of parts.
Without knowing part volume, size (3" x 3"?), or wall thickness, my best guess would be that a machine in the 75T to 100T range would probably be right.
ABS and acrylic are not tough to mold. Any injection shop should be able to run them.
Take a look at this thread:
http://cnczone.com/forums/showthread.php?t=8239. It's one from a while back and it should answer some of the questions you have.
Chris Kirchen