Welcome to the forum and to Mini 14 Ownership. I'm a new Mini owner myself as of September and I've put about 1000 rounds through my Mini so far. It's a great shooter and a ton of fun at the range. I have shot a wide variety of ammunition so far without a single hiccup. I've used Brown Bear and it shoots perfectly in my Mini. As does WPA, Wolf, Tula, Remington, UMC, Federal, Winchester and Freedom Munitions. It'll take the 5.56 or the .223 all day in steel or brass and doesn't flinch a bit. As for warranty issues... I dunno. I never read the manual. I guess I should though.
As for magazines, I'm a believer in the phrase, "You get what you pay for." My advice? Pony up a few bucks extra and get the Ruger magazines. Everyone I've talked to will tell you that the Mini 14 loves Ruger magazines the best. They're not that much more expensive than the aftermarket counterparts and they work every single time. I have three of the 20-round magazines and a 5-round magazine. I rarely use more than one magazine even when out at the range. I'll typically shoot 20, stop and reload and converse with my range buddies and then get back to shooting. If I'm out at the family farm or something, I'll load up all three of them and have at it, but the outdoor range I'm a member of isn't set up for me to just go blasting away at anything I want to so I typically shoot a little slower and just use the one. Get one or two Ruger magazines now and a couple later. I'm planning on ordering another few after Christmas just to have because there's a chance that they'll get hard to find at some point and I'd like to have a handful of them in my range bag for just such an occasion.
As for the grip... I dunno. Hopefully someone else can come along and help you out. I know a lot of people are fond of the Tapco stocks and they should be able to get you pointed in the right direction.
Hope this helps you out. Enjoy that Mini!!!