* rizla used to design military radar/ESM/ECM systems
What you are proposing is unfortunately totally illegal in the UK. Nothing to do with actually tracking targets but more to do with transmitting unlicensed signals on restricted bands. The unlicensed bands are generally so full of noise that you'd run into problems (easily if not cheaply fixed) and there are still maximum ERP limits too
Magnetrons are still the transmission source of choice for most commercial radars and with them comes some fairly high dc voltages, usually requiring a bit of inverter design - hence some largish caps.
On the receiver side you can in fact discriminate signals well below the noise floor which will be inherent in any RF design.
A more practical (if not cheap) design would be to cobble together a couple of RF amps, a DIFM (digital instantaneous frequency measurement) unit and a few interface components so the box could talk to a PC. Then write a bit of code to display the raw output on the PC in a prettier format. You'd need a radome and some waveguide too. With that you ought to be able to see emitters and to some extent track them as well. Eg you could probably track commercial aircraft via the rad-alt and IFF systems.
Transmitting microwaves at moderate to high power levels (>1kW) is generally not a good idea unless you know what you're doing. I wouldn't want to live next door to someone doing what you propose