Yeah; for assessing which cyber-limb accessories require surgery I'd base it off the essence cost (including a DNI link as discussed) from the ECU table.
M&M p35
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EQUIPMENT CAPACITY
Replacement limbs offer a certain amount of space that can be used to accommodate additional equipment. The amount of space available depends on the limb and is different for obvious cyberlimbs and for synthetic. The space is measured in units of equipment capacity. Each unit offers roughly 50 cubic centimetres of space.
The Equipment Capacity Cost Table lists the number of equipment capacity units various implants and devices require in order to be installed in cyber replacements. Most additional equipment can be installed in cyber-limbs without Essence cost. Equipment that must interface with other, non-cyber parts of the body, however, does cost additional Essence, as shown on the Equipment Capacity Cost Table. For equipment not listed on the table, the gamemaster can establish an equipment capacity unit cost based on the gear’s approximate size, using the Equipment Capacity Units Table. Unless the equipment description includes it, most devices installed in a cyberlimb do not have a direct neural connection to the brain. In order for the user to cybernetically control such a device, it must be installed with a direct neural interface (see p. 39). These items are marked with an asterisk under Essence Cost on the Equipment Capacity Cost Table.
Relevant skills for installation.
M&M p135
[ Spoiler ]
Computer (Cybernetics)
The Cybernetics Specialization of Computer Skill includes knowledge of any cybertechnology that incorporates computerized systems, whether hardware or software. It focuses on the neural interface between such devices and the brain and nervous system, as well as datalinks to other devices.
Computer Cybernetics B/R: This Build/Repair Skill is called upon by implant surgeons repairing headware, skillwires, matrixware, riggerware and other computerized systems.
Electronics (Cybertechnology)
The Cybertechnology Specialization of Electronics deals with cyberware that incorporates electronics, especially senseware, cyberlimbs, smartlinks and other powered devices. Power and control systems, diagnostics, flux and communication, motorization and micro-engines are covered.
Electronics (Cybertechnology) B/R: This Build/Repair Sltill handles the construction, maintenance and repair of electronic cyberware.