Welcome in the fascinating world of PSoCs!
1. PSoC3 and 5 are equal except that PSoC 5 has got a modern ARM M3 core and PSoC3 an 8051
2. No, GND <= Vin <= Vcc
3. There are some opamps you may support externally to have a filter
4.There is a multiplying mixer component
5. I cannot tell.
6. External clock is only needed when you want to have a precise timing.
To get more information and to get acquired to PSoCs I would suggest you to download and install the Creator from the Cypress website. Thie IDE has got a component catalog with detailled datasheets for each of them. There are many example projects, too.
Depending on your finacial status there are some different development kits starting from $4 with a PSoC4 to $250 for PSoC 1,3,4,and 5 including a MiniProg3 as a Programmer and a debugger for PSoC3, 4 and 5 chips.
1. Am I right to assume PSOC 5 have the same or better capability than the rest of PSOC 1,3 and 4 in terms of analogue and digital capability?
Not exactly, PSOC 1 being switch cap (SC) based (and some CT based) analog has capabilities that can
be worked around in 3, 4, 5LP. For example, using no MIPs you can easily create using SC various filters.
In 3, 4, 5LP only one pole available, passive, or use DFB + A/D + DAC. Or create SC using muxes in 3, 4, 5LP,
limited to external components for passives.
3. Can I directly implement analogue low pass, high pass and bandpass filters with PSOC 5?
"Directly", if that means in HW yes because of the DFB, DAC, A/D capability to create digital filter. IIR
or FIR or using DFB assembler FFT, comb....
4. I have found application note stating how to implement analogue multiplier with PSOC 1. Can I do the same with PSOC 5?
DFB approach one way, as Bob stated there is a mixer component as well. Or A/D, DAC + computation.
Regarding SPI: I don't think you get a speedup here. The SPI component is limited by the internal delays of the UDB and the data path. And these won't change with the frequency of the MCU core.