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Depends what type of competition you are doing. Circuit racing, they do need to be plumbed in and discharge into both cockpit and engine compartment as a minimum and be accessible from driver's seat, as well as an external marshall's pull. Rallying also requires a handheld for co driver. You can get cheaper mechanical devices from about £120 iirc but electrical as above are better.
Don't bother with any of the electric battery isolators, its not uncommon for them to fail and cause you issues. If you want an electric one a lot of people just get a manual isolator and hook it up to a central locking solenoid these days.