Can you maybe explain the differences in the different Acorn machines and whether they are compatible with each other?
BBC Micro - 8-bit machine. Fast for it's time.
Electron - Cut-down version of BBC Micro. BASIC is compatible with BBCs, but is slower.
A3xx/A4xx/A3xxx/A4000/A5000 - First gen Archimedes machines, as Bloodwych described above.
A7000/RiscPC - Second gen Archimedes machines, with enhanced graphics abilities. Mostly backwards compatible with first gen Archimedes.
A cheap way to get hold of Acorn machines is from schools - mine is throwing out loads of RiscPCs and A5000s, and I'm going to get hold of most of them.