Its easy, especially if you buy the battery. But you mentioned playing Zelda Ocarina of time. I dont think there is a good N64 emulator out. I have tried a few and none of them work all that great. Some will play the games full speed for a few mins then crash, or they will play the game at almost full speed but you cant have sound. But with cfw you can do sooo much, so even if you cant use a N64 emulator, theres more out there.
Yeah, the N64 emu is still being worked on, so maybe someday OoT will play nicely on it, but right now it doesn't. Super Mario 64 and Mario Kart 64 are really the only games that are quite playable.
You can hardmod it yourself. It's really not that hard. Google it. Basically you cut the battery open and cut a small piece of metal inside it (don't worry. It won't blow up. Not unless you violently stab the battery pack I guess. =|)
Here's the extended battery on amazon. www.amazon.com/PSP-Battery-Kit-Sony/dp/B00101UWWY/
I've got two of these (note: I bought one in a best buy and one in a gamestop, but this looks like exactly the same thing to me) and they are definitely worth it.
I have a modded psp slim. And I love it. If you need any free games or homebrew apps just let me know. Instuctables is a good place to look if you need any help.