It's not really a kit, but Daryl (8BIT) here on the forum makes a little 4x5" hobbyist SBC board available that takes a 6502, RAM, EPROM or EEPROM, a 6551 UART, RS-232 line drivers and receivers, two 6522's (each having up to 20 I/O bits, a synchronous serial port, two 16-bit timer/counters etc.), reset circuit, crytal can clock oscillator, and four 14-pin pin headers for I/O. You supply the parts that go into the board, and connect all your own I/O. That might include for example a keyboard, LCD, other ICs, a model railroad setup, or whatever you want to interface it to. I don't remember that he supplies any standard software for it, but he'll correct me if I'm wrong. You'll just need a power supply, (E)EPROM programmer, and assembler (although some of us started out years ago assembling by hand).
http://www.6502.org/oldmicro/trainers.htm may be of some help too.
When you have questions, come back to the forum again and ask. There are some very knowledgeable people here who'll be glad to help. We would encourage you however to see if the same question was already asked and answered before so we don't clutter the forum with the same things over and over.