It's not clear what you consider accessories, but here's a start:
Your local regulations will require a variety of lighting systems, and probably windscreen wiping and demisting systems, unless your car has an open cockpit with a transparent "wind-deflector" screen, like some Lotus 7 clones.
Depending on your choice of components, you may need an airpump to create vacuum or positive pressure for your brakes and your steering rack may have electrical assistance, or hydraulically-powered assistance using a pulley drive which you'd need to drive somehow.
You need a speedometer and you should have a battery voltage/level gauge. If your drivetrain has multiple ratios, you'll want a tachometer (rev counter) to avoid exceeding the motor's rated speed when downshifting.
You should probably, no definitely see what your part of the world calls for, some may require you to have all the factory safety systems working like ABS, Airbags, some may be age dependent depending on what vehicle you choose. If you are trying to cut down on expended using energy on other systems then only you can decide what you need. For me pack cooling and heating is not required, but for others its a must. I don't need power brakes or power steering but again for others that's a must have, legally and personally.
Most countries have a government list of regulations.In AU NSW state, the authority is Roads and Maritime Services. They have an online list of regulations for all types of conversions and also a list of recognised engineers and their locations. My first conversion was a 1999 Daihatsu Sirion. The main issue I faced was circumventing things such as airbag, seatbelt and brake warning lights. The more modern the car the more technical the conversion. My two conversions since that have been a 1989 Suzuki Carry and a 1985 Subaru Sherpa, both of which only needed brake fluid level lights to be maintained. The problem in Finland could be getting an older vehicle without rust. But if you are a computer whiz then you will have no problem.
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