Everytime someone mentions the a Sinclair and accounts, I always think of Hey Hey 16k - http://www2.b3ta.com/heyhey16k/
The practical difficulties such as very restricted memory (the ZX-80 had < 1kB of usable RAM), unreliable cassette tape drives, long loading/saving times and non-ergonomic keyboards meant that even the most dedicated business user would struggle to use them for any serious purpose.