Looking for statistics on the Computer market of the 80's

OK, let's see, how is possible to change the CPU of the ZX Spectrum 48K (the rubber keyd version) to get double/triple the performance? how could I have no 48K but 1MB instead? How could I mount a CDROM or Hard disk? Could I change my old floppy drive( no "sinclair" floppies existed for the ZX48K) to a new larger one? Let's say, I need a better resolution on graphics, and 3 channel sound generator. Upgrading from my point of view, and I guess you would agree with me, is something that can be done by (almost) everyday people, at very *VERY* reasonable cost, and not by nerds or extreme hobbyist (like myself). "Upgrading" should not need an Electronic Engineering degree. Only modular systems, and with a lot of hardware options and development through time, could be considered "Upgrade-friendly", like the PC or the Apple-II. Amigas were a different case, I am reffering mostly to 80's 8bit home computers, and not the 16/32bit monsters :-)