1) For sensing RFI from PCBs etc, what kind of wideband preamp is suggested?
Probably none.
Gain in front of the first mixer can cause overload problems.
Leave the wideband amp until you have the rest of it working.
2) Are there combined VCO or (pref. I2C/SPI controlled) oscillators that can cover the freq band of interest? Hopefully they can also produce the IF by difference mixing?
As previously mentioned, a 1:100 frequency ratio is a bad idea and unnecessary anyway.
Make the first IF frequency of your analyser higher than the highest input frequency.
For example, if the first IF of the analyser is 200Mhz, then the first local oscillator only needs to run from 200 to 300Mhz and the analyser will tune from 0 to 100Mhz.
A VCO running from 200 to 300Mhz is relatively easy.
3) By linking the VCO sweep period to the 'scopes trigger I should be able to view the various freq. amplitudes on a single scope channel?
Yes, that is what the average spectrum analyser does.
Infact it uses the timebase oscillator output as the drive for the VCO.
4) By injecting (mixing) known period, small duty, fast square pulses of peak vertical amplitude (based on the VCO cycle period)matching the scopes display I can create horiz. 'scale' markings (vertically) on the CRT of known frequencies to 'calibrate' the CRT freq. display?
Once the analyser has been calibrated and assuming that the timebase and VCO are linear, the 10 x-axis divisions on the scope graticule will represent 0, 10, 20...90, 100 Mhz.
a Full time calibrator is not necessary or even desirable.
eg. If the CRT is displaying the full 112Mhz Bandwidth of the VCO, injecting pulses every 1/8 VCO period should create vertical lines on the CRT @ 14Mhz, 28Mhz , 42Mhz .... 112Mhz; for a 112MHz VCO bandwidth?
Sounds like a bad idea.
5) If #4 is reasonable, can I then create those same calibration pulses alternating at 2 known amplitudes after each VCO period thereby creating an apparent vertical amplitude reference for aligning the vertical deflection of the scope for more accurate amplitude readings?
Eg. Pulse 1 @ 1V, Pulse 2 @ 2v, pulse 3 @ 1V ...etc
6) #5 can be expanded to multiple known amplitudes if a log. output is used to help visualise the signal amplitudes?
Use several stages of logarithmic amplifier, usually as the lasy IF amplifier, calibrate it and you have your vertical scale calibrated in dB.
JimB