I was not so nice to my first one, expanded memory and added a vt100 serial port. But still have it and 2 other kits I never assembled (OK never had time to).
Also have Nascom, TRS80 and Kemitron systems... Sigh the good old days
If you have a pristine unassembled MK14 with all the bits, then it's probably worth a lot of money. I saw an assembled one (pristine) go for $3000 some time back, and a one about as grubby as mine sold for £450 a week ago on ebay...
I built my own 6502 based board after that, (wire wrapped!) then went down the Apple, Beeb route... wish I'd kept the North Star horizon, but it was just too heavy...
I had an SD Systems Z80 Starter Kit . It had 2kb of RAM and an 8kb EPROM socket. The board was etched for two S-100 bus connectors but you had to buy the sockets separately. Of course, you had to solder all of the components to the board yourself. I used it mainly as an EPROM burner.
I also designed and built my own Z80 "gray box" computer on a 6" by 8" "breadboard". It was used by a local bank to translate 9600 Baud ASCII to/from their fancy Phillips wordprocessor to their 150 Baud 5 bit Baudot code teletype machine. The secretaries would construct fancy International Letters of Credit on the wordprocessor but they had to be transmitted via secured Western Union teletype links.