Welcome to the group! Here you will meet interesting people and learn interesting things.
The parts you'll need depend on what kind of Elf system you are building.
I assume you are familiar with the "classical" Elf, with the 8 switches and 256 bytes
of RAM. If you do the switches, may I recommend rocker- or paddle switches as they are
easier on the fingers.
The BG kit includes a large RAM and an EPROM (presumably blank). To use these you will
need a high-address latch eg.74HC373. Usually these systems place the ROM at #0000 and
the RAM at #8000, sometimes these addresses are reversed. There are reasons for each setup.
Go to Herb Johnson's website and read about Lee Hart's "Membership Card" Elf kit.
It's an interesting design example to show how to set up a system with EPROM and large RAM.
If you want a serial monitor setup, Lee can supply a EPROM containing the IDIOT monitor.
This is a good choice for a first system. The serial I/O uses the EF and Q signals and a
software UART to keep the hardware simple. You'll need a MAX-232 chip to interface the EF
and Q lines to a PC's serial port, and run a terminal program to talk to IDIOT.
This will allow you to load and save memory and examine and change the CPU registers.
Many folks on the group run systems with video and sound hardware, cassette storage,
hardware UART serial and other advanced peripherals. There exist assemblers that run on the
PC so you don't have to assemble with pencil and paper. (I've done that)
Some of us design systems with vintage parts, some prefer the modern parts.
Retrocomputing is as much a work of art as it is electronic design.
Herb Johnson's and Lee Hart's websites contain a wealth of knowledge about the COSMAC;
and searching the Web you'll be able to find Ipso Facto, Questdata, and the Viper.
as well as many original RCA documents about their COSMAC products and development
systems. These publications from back in the day are a goldmine of information about
the 1802 and the systems then built around it.
Happy New Year!