The standard installation depth for Dell equipment is 28 7/8". This is the distance from the front upright in the rack to the rear upright (where the square holes are).
PE R710 has a mounting depth range of 27 1/4" to 29 3/4" and an overall depth of 35 3/8"
PE 2950 has a mounting depth range of 26 15/16" to 29 5/16" and an overall depth of 35 3/16"
PE 1850 has a non-slide rail design completely different from the PE2950 and R710.
Definately choose the 36" deep rack and make sure the installation depth is set at 29". If the rack does not provide square hole mounting you will require an alternative rail kit for the R710. Give me a call if you have any questions.