I keep my modified IDX Pro with me in my rig all the time. Sometimes when I head out I include one, two or maybe three other detectors, but the IDX Pro is always tagging along.
"Why is there no mention of the IDX PRO.".. Oh but there is, and has been through the years. Sadly, White's discontinued it last year but it continues to be a sought after model ... and for good reason.
"Is it a good detector?"... In some applications it is considered to be a 'GOOD' detector, yes. But in many, many other applications I rate the IDX Pro as 'EXCELLENT.'
"What did it evolve from? What did it evolve into?"... About 1994, White's brought out the Classic III Plus. A great non-metered model that had a selector that enhanced the discriminate range lower to accept all metals. This is labeled the "Normal/Black Sand" toggle but, in my opinion, should have been named "Normal/Enhanced" because that's what happens when you toggle to the Black Sand setting. You "enhance" or "expand" the discriminate adjustment range lower.
1996 Saw the Classic's, formerly housed in the more squarish housing and powered by 3-9V batteries, upgraded to the newer "Slim-Line" housing as was used in the XLT. These new 'SL' model Classics were now powered by a drop-in 8-AA battery tray. The Classic III remained the same in performance and features, but had a better 'feel' and balance, and the handy trigger-toggle.
1998 White's introduced the Classic ID. The basic Classic circuitry had been developed through a contract with William Lahr. White's took the basic Classic II circuitry with 2-knob operation, and John Earle, an excellent engineer working for White's developed a Target ID circuitry using "Flash Phase" technology and incorporated it with the Classic II design.
It seems like it was about 1999-2000 when White's introduced the Classic IDX. This model used the same "Flash Phase" technology Target ID circuitry as the Classic ID, but it was incorporated with the basic Classic III SL circuitry, with one improvement on the upper-end of the discriminate adjustment range.
In 2001 White's brought us the 'Pro' series (again) that was simply a renaming of three models. The Quantum XT became the QXT Pro, the 6000 Pro XL became the XL Pro, and the Classic IDX became the IDX Pro. Other than a name and decal change, the ID Pro also got a slight increase or boost to the upper-end of the Sensitivity adjustment range. It also got new decal for the coil and below the TID display that had a matte finish to reduce the glare from the shiny decals used on the 'original' IDX.
From there it "evolved" into a discontinued model. The basic Classic circuitry wasn't used in anything else for the hobby line as a regular White's dealer product.
"I see a few for sale, but never by the owner, always by 3 party sellers. Please let the Gods of Detecting give me knowledge on this unit!"... That's about the best info I can give you, but it comes with some encouragement to give one some serious consideration.
These are excellent slow-motion, quick-response detectors that will give you about 65-70