1. Yeah, the MAX203 should work. It seems to be pretty much like the MAX233. It still needs the bypass cap and other wiring, like it shows on the datasheet.

2. It's speed-independant.

3. I got a bunch from here: http://www.sealiecomputing.com/retrozone/

4. I think you'd wire all the pins to both chips to the same socket, except you'd have to set it up so only one chip's output is enabled at a time. I dunno how to have it default to the ROM, though