With regard to the temp gauge failure, did you verify that the actual gauge is the culprit? A bad connection at the bulkhead plug or at the dash ribbon cable could easily provide a no-temp signal condition on the gauge.
On the issue of angle drives, Rob Grady makes a great angle drive. Make sure your lower cable turns freely and isn't gunked / rusted up. Based upon the amount of time your car has been sitting, lower cable rust can be an issue. I recently disassembled 4194's lower cable and gave it a good cleaning.
While I would certainly agree that the DeLorean's speedometer system is far from perfect, there is nothing inherently wrong with a mechanical speedometer system. They worked just fine from the early days of speed limits all the way until the late 80's / early 90's. I suspect that the changeover from mechanical to VSS systems was more the result of advances in ECM / TCM technology and the need to electronically monitor vehicle speed than any problem with mechanical speedometers. Interestingly, my 1985 Toronado's digital dash gets its speed signal via a mechanical cable and an optical pickup. :-)