Short answers, pulled from the story.
The ZX Spectrum was officially revealed before journalists by Sinclair at the Churchill Hotel in Marylebone, London on the 23rd of April 1982. Later that week the machine was presented in a blaze of publicity at the Earl's Court Computer Show in London and the ZX Microfair in Manchester.
The ZX Spectrum launched with two models including a 16KB basic version priced at £125 and an enhanced 48KB variant priced at £175. The former model had an undercutting price significantly lower than its main competitor the BBC Micro while the latter model's price was comparable to a third of an Apple II computer.
By the end of the ZX Spectrum's lifespan in 1992 over five million units had been sold making it one of the best-selling British computers of all time. It retained the title of Britain's top-selling computer until the Amstrad PCW surpassed it in the 1990s with eight million units sold by the end of the PCW's lifespan in 1998.
Chief engineer Richard Altwasser was responsible for the ZX Spectrum's hardware design which included a semi-custom uncommitted logic array integrated circuit. The team agreed that the ZX Spectrum must have high-resolution graphics and 16 kilobytes of memory before launching the machine.
Sinclair Research granted a licence for the ZX Spectrum design to the Timex Corporation in the United States who introduced an enhanced variant known as the Timex Sinclair 2068. This upgraded model features improvements in sound and graphics but Timex's versions were generally not compatible with Sinclair systems.