A4 is the size. In Europe it is A4 (210mm x 297mm)in the US it would be considered an 8.5" x 11". Not quite the same size but to make it simple, they are referred to by their European size. An A3 is much bigger than the A4 and they keep going up A2, A1 etc. The A3 is more common in the DTG market because its size is more compatible with printing on garments. The A4 will work but you are limited on size.
RIP stands for Raster Image Processor. It is a software used in the professional printing industry to control individual ink channel outputs to produce accurate and repeatable results. It is used in the DTG industry for two reasons. First, it is used because you can increase the ink output in order to produce more vibrant prints. When printing on fabric, the fabric will absorb a certain amount of the ink before you actually see your printed results. This is simplifying it but basically you put down more ink to compensate. The other reason DTG uses RIP is to allow printing in White. None of the home models of Epson printers come with a white channel. However, the better A3 and above printers come with a second black. One is gloss and one is matte. By changing one of these over to white, you can turn every other channel off when printing a white underlayer on dark material and then turn off the white and turn the others back on to print the color over the white. Without this ability you would not be able to print on dark fabric. I have read in some instances where you can simply choose to print in black and white using a particular paper choice and it will only use the white channel anyway. This would take some trial and error but I can see where it could work. For now I wouldn't worry about this because you won't need it for white since your R200 won't print white and until you have a working printer, you don't need to spend the money on a RIP. There is a free RIP out there that probably supports the R200 called Gutenprint but it is old and unsupported so you might find it challenging to get it to work.
My R230 was purchased from China as a flat bed hard surface printer. I have had it apart multiple times for repairs so I am intimately familiar with how it works. The best advice I can give you is as I said before, go to the pinned thread http://www.t-shirtforums.com/diy-dtg/t32499.html
. This guy uses the C88 which is mechanically very close to the R230. He has a PDF that does a very good job of explaining his conversion. If you follow that you should be in great shape.