Any time you change the pads or otherwise alter the system, it's necessary to bleed it. I bled/flushed my system using ATE Super Blue DOT4 fluid when I added my Goodridge SS lines at 30k and again when I changed from stock to Hawk HPS pads all around at 53k.
I did not resurface my rotors when I changed my brake pads at 53k miles (fronts had ~25% life left, rears ~50% left) because I didn't see or feel any deep gouges in any of the rotor surfaces and the brake pedal wasn't fluttering at all. Looking back, it probably would have been a good idea to have them all resurfaced just because it would have provided a fresh surface for the new pads to bite in to and it might prolong the life of the stock rotors, but I kept telling myself it didn't matter because the next time I serviced my brakes wouldn't be for a long time and by then, I'd probably want to replace the rotors with something cool (cross drilled and/or slotted... yeah I know).
You do not have to replace the brake lines if you're just replacing pads, but many people do them at the same time since you're disturbing the system and you'll have to bleed the brakes anyway. People upgrade to stainless steel brake lines because (1) they provide a slightly firmer pedal feel when braking, (2) aftermarket stainless steel lines are more durable than stock rubber lines which are prone to cracking over time - which isn't really a valid argument because the stock rubber lines will often outlast the life of the entire car, (3) it's cool to say you have stainless steel brake lines on your mod list, (4) if anybody looks that close at your wheel wells, they'll see something slightly shiney instead of something black, and (5) it's a relatively inexpensive mod at $100 +/-.
You probably already know this, but when you're finished, spray any surface that brake fluid might have contacted with a bottle of water and wipe it off really well with a clean rag because brake fluid is very corrosive and will damage any painted surface if left untouched. Also, spray the surfaces of your rotors with Brakleen if you happen to touch them. If you haven't already discovered Nitrile gloves, buy some - they're so much better (more durable) than Latex gloves and they'll keep your hands squeaky clean when doing a dirty job like messing with your brakes.
P.S. - Bleed order for the WRX is:
1. Front Right
2. Rear Left
3. Front Left
4. Rear Right