For 'traditional' car wiring (eg 1970s bullet/spade etc) and up to 30A wiring I would have thought an inexpensive 40W iron would be fine. And ordinary lead free fluxed solder:
eg:

https://www.toolstation.com/draper-soldering-iron-stand/p86557
https://www.toolstation.com/lead-free-flux-cored-solder/p97854

or with slightly more control - a manual temp control iron: https://www.toolstation.com/draper-soldering-station/p62910

For more intricate stuff on pcbs say, or with delicate components, it is well worth getting a temperature controlled iron, with a mix of different bits. Professional units will cost £200 plus,

https://uk.rs-online.com/web/c/?searchTerm=weller+soldering+station

(While you might think you don't need a high power set up (eg 80W) for small components, that can be important if you are rapidly soldering a number of locations at once (eg a 40 pin IC). And a high wattage can also be useful if the thing you are soldering has a large thermal capacity (eg a bit lump of copper say).

Investing in a good desoldering tool (eg something that will suck molten solder off a pcb or a connector when you are undoing something) is worth considering.

I've just suggested some examples, but I'm sure there are plenty of equally good alternatives in the same price range.

Dave
Blue 4/4 1969, Green +4 1953, (different) Green +8 1977