To the best of my knowledge there are only three types of hardmod for the 3ds
1) NAND dumper/flasher
2) Video capture
3) Control redirect/turbo/outside inputs/autoplay inputs (or I guess control reads).
I am not sure if there are still kits for 2) either -- most seemed to want you to send the thing off.
1) is fine but I am not sure what you are going to get from it -- software dumping is possible with a 4.5 system. Should you hose things up hard enough to need it then I would solder in the thing to flash the 4.5 dump back.
I guess there is a slight variation on the theme if you somehow went back and bricked your 3ds with the early gateway/gateway derived stuff.
About the only real use then would be if you have a later system, say one on 9.2. If gateway's new exploit is indeed stopped by 9.3 then you could dump the 9.2 for your 3ds, use the new firmwares for eshop/online and then downgrade when it gets released.
I am not sure what there is in the way of dual nand right now but if you get a 4.5 system then it is all in software anyway.
Specific soldering irons... you are doing close soldering on small pads/pins/traces on a modern multilayer board. Plan accordingly. I like hot air for this sort of thing but you can definitely still use a soldering iron.
Conductive glue. Not a particular fan of the stuff myself, or at least not the stuff you can buy in shops*, but there is little need for a permanent mod for the NAND stuff, to that end it has its uses.
*a friend once did a sort of money basically no object type test to see if they could make some of it, the results were impressive.
I guess you could invent another mod (memory dumping/tracing/fiddling might be nice) but if you want that and are asking these sorts of questions then it not something I would suggest.
