If you have a fine-tipped soldering iron and a steady hand, you should go for it
It's not that hard, and even if you make a mistake, you can just grab your solder-remover and start again. Yes you definitely need to add the resistors yourself, because these plugs come with nothing but the pins, pcb and plastic shell. You'll need 27 ohm resistors for the P model or 102 ohm for S. If you're off by an ohm or two, it's no big deal (it will be easier to find 100 ohm resistors!), but you need to match them for the left and right drivers, to make sure you get the same frequency response from both. You can do this by buying a huge pile of resistors and testing them all with a good multi-meter to find a nice matching pair, or (my preference) is to just buy some good quality low tolerance TFT resistors, which already come with the exact same resistance and are tiny enough to fit in the plug housing.
As for cables, I'm a fan of anything that reduces microphonics or adds functionality (I know some see this as sacrelige, but I like to use my ER4's with my iPhone and really need a remote!). For either of those two reasons, it's totally worth it. However - and I'm not wanting to get into a flame war here - IMHO, if you're only looking to 'upgrade' your cable by soldering on those highest-purity silver/unobtanium cables, you're wasting your time and money. Any cable that gives you the necessary impedance will do the trick. Of course, the FR will change if the cable impedance changes. You might even prefer that change, but it isn't magical, it isn't what the manufacturer intended and it isn't something you couldn't have achieved via a simple EQ. (Check out the Pencil EQ in the 'Can Opener' app for iPhone - it also works with external DACs. It's awesome!) I suspect slight changes in impedance have convinced some people that those $800 cables on Amazon and eBay are worth the money. Unless somebody shows you some newly-discovered electro-magnetic-gravitational-relativistic physical property of cables that we didn't previously know about, I'd view such cable 'upgrades' as very expensive jars of snakeoil