Hi Guglielmo,
What you'd need to play the Tape Project tapes and 1/4 track tapes like you find on auction sites would be a 1/2 track and 1/4 track (or 4 track) heads and the ability to switch between 7.5 ips/NAB eq for the 1/4 track tapes and IEC/1/2 track/15 ips for the Tape Project tapes. The ability to switch between NAB and IEC would be nice since some 15 ips tapes you might want will be printed with NAB eq.
I believe that most all Studer machines will allow you to switch between NAB and IEC and most should have both 7.5ips and 15 ips. You should double check this with any machine you're considering since some speed conversions aren't trivial.
Regarding a Studer with two playback heads, while I'm sure that most machines would have had many options regarding the head configurations those would be considered a "custom" order.
On the other hand, you could have JFR modify or custom make just about any head configuration you could dream of as long as it will fit. In other words, you may have to get one where the extra head would take the place of the record head so it would no longer be able to record. You could also get another headblock for 1/4 track playback. If you had both playback heads in the same headblock, you'd also need to have JFR install a switch for 1/2 or 1/4 track playback.
Much of this would depend on the model of Studer you're interested in. You should consult JFR to find out.
IMHO, most 1/4 track tapes don't deserve such a royal treatment. Sure, there are some (like the Barclay-crockers) that might, but overall, the quality of the duplication and the wear and tear of the decades outweigh any benefits of such and expense.
You'd probably be better off buying a second machine (or saving one that you already have) and using it for 1/4 track tapes and dedicating something like a Studer for 15 ips/1/2 track tapes.
It certainly couldn't hurt to inquire about the cost of having JFR do the custom headblock work. And, if you're planning on getting a really nice machine, I would include the cost of sending the headblock to JFR for a check if there's any perceivable wear on the heads anyway.
http://jrfmagnetics.com/