The issue of 240V vs 12V, or low voltage vs extra-low voltage has been discussed here plenty of times.
(Those are the proper terms , BTW - when used correctly "low voltage" means up to 1000V AC or 1500V DC between conductors and up to 600V AC or 900V DC between any conductor and earth. Extra-low-voltage means up to 50V AC or 120V DC, but in practice 99.999999999999% of ELV products on the market are 12V lighting systems.)
There is no doubt that ELV is "brighter" than LV, but the differences are not as great as some people make out, and the lamps tend to be more expensive, but again the difference is not as much as it once was. Let's put it this way - if you were concerned about efficiency or running costs, you'd not be considering
any sort of halogen downlighters...
Everything being equal I'd agree with your choice of LV, but if you see a luminaire that you just
have to have, and it's ELV, then go for it.
Regarding your general questions, I get the impression that you're not that experienced at electrics, so you should get at least the first book from this list:
//www.diynot.com/forums/viewtopic.php?p=75416#75416
and I'd strongly advise numbers 2 and 3 as well. You need to make sure you understand how lighting circuits are wired.
And buy a multimeter.
And make sure you fit fire hoods or use luminaires with inbuilt fire protection if there is a habitable room above the living room.
When cutting holes, be careful of the dust, some Artex used to contain asbestos...