I have a C128DCR on my bench under my monitor that I fire up about once a week. I've modded it a bit too.
DCR is different from the c64 style 'keyboard' only original C128.
video output on the c128 is a difficult thing. Understand the C128 has two video outs, its original c64 compatible tv output / 40 col mode, and a hires 80 col mode. Both separate plugs on the back. The 80 col mode is modified RGBI signal.
Best thing is to get on the C128 monitors, eg: 1902A or 1902 (one is ntsc, one is pal) or just use the plain old TV out signal and stick to 40 col mode.
One thing to note, first get C128's didnt have the video ram fully populated. A common hack is to up it to the 64kb it is supposed to have from the 16kb it comes with. You can find the pcb's on ebay for cheap, plugs into the socket, or probably just solder your own ram chip.
Something you need to get is a SD/uIEC so you can do disk images off an SD card without using a real floppy disk drive. Thats the cheapest way to do SD card disk images. the more expensive but better way is an Ultimate u1541-ii cartridge which basically emulates a disk drive properly (since the disk drive has a real cpu, ram and os inside its not a small thing to emulate).
http://store.go4retro.com/uiec-sd/any questions, just ask!