Scientific Linux. http://www.scientificlinux.org/ Has the benefit of RHEL: a stable OS environment without some of the headaches of CentOS. If you have money (you probably don't) RHEL is good.
Turing-complete means that it is able to perform all of the functions of a universal Turing machine, not that it is able to solve the Turing halting problem; a Turing-complete language (or system) by definition is unable to solve the halting problem expressed within that system.
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