Best elements for downhole and high temperature circuits
I don't have borehole experience, but I do have a lot of generalized experience, including space environments.

First, temperature. PICs won't do it. Go look up some data sheets. Their max is 85 C. You need MCUs rated for automotive or full mil-spec temperature (125 C). The same applies to all other components - full automotive (not industrial) or mil ratings.

Second, pressure. That's easy, no special pressure requirements on your circuits. The reason is simple: if you're going to operate the circuit at ambient pressure, you have to allow ambient materials into your electronics. And water and drilling mud are not what you want inside your data logger. This leads to the biggie:

Third, housing. You must learn to build hermetically sealed housings for your units, and they must withstand temperature and pressure. Think lots of stainless steel. And finally,

Test facilities. If you do not have the ability to apply a good replica of the operating conditions (temperature, pressure, material), and if you do not use this ability to run prolonged stress tests on your product, you are doomed.