Some of things which made the PRO-2006 so popular were
- easy access to tap the discriminator output (useful for DIY experimentation)
- ability to restore the "forbidden" UHF analog cell phone frequencies
- ability to increase the scan speed (replace the 12 MHZ crystal with a 16 MHz one)
- extremely wide coverage (25-520 MHz, 760-1300 MHz)
- overall design & ease of use (IIRC, they were made in Japan for RS by GRE, who were tech leaders at the time)
I recall one of the more extreme modifications involves expanding the memory to 6400 channels - it involves replacing (& upgrading) the memory chip and installing a 4-position DIP switch to switch between 400-channel banks.
While it doesn't support trunking or digital operations, it's still very useful for analog scanning; there's a lot of information available out there on them.
For an antenna, you've a lot of inexpensive and useful choices, from a whip with a BNC connector that connects directly to the scanner to tower-mounted log periodic antennas (useful but not inexpensive)... Since we'll rule out the expensive options, I'd look for discone plans or modification of an old TV antenna to feed the scanner; 50 ohm or 75 ohm coax work equally well for this purpose. (A whip with BNC connector is handy to have in any case, though, if for nothing else than to be a handy reference antenna when trying out other antenna / feedline combinations,)