A uMP2000 is an upgraded MP2k. Either Apple or someone like Dr. Newton or PixSolutions upgraded the RAM (user RAM, not storage) to 4 MB from the 900 some k it had. Mine was done by Apple and has a little "MP2100" sticker by the interconnect port. The added RAM sure is nice, as I have had a regular 2000 as well.
As for the external "upgrades," I would recommend a flash card for sure and a modem or ethernet card (although I don't know if ethernet works on 2.0 -- maybe only 2.1) depending on your situation. Talk to a local ISP, as the flash cards they use for their routers can be used in the Newton for some time after they are beyond use to the ISP.