I'm just going to throw out some thoughts here, so please bear with me:
1) A 6" group at 15-yards isn't an insurmountable standard. However it is a bit tighter than most marksmanship tests I've heard of. There's a local indoor range here that requires you to place all shots on the paper of a standard NRA 50' pistol bull before you get signed off. (Note: not all in scoring, just visible on the paper as I understood it.)
2) More ergonomic, "better" designed equipment does make the shooter's task easier. No offense at all intended to those who own them, but I've never seen anybody shoot well with a Hi Point. For a $300-400 budget I'd take a long, hard look at the new Ruger SR-9E. A friend has a stainless SR-40 that has made me rethink my stance on the .40 S&W. At 10-yards I was able to plunk 5 shots in the black of a 50' NRA pistol bull with one hand. And that was about the 4th magazine I'd ever fired through it. Very impressive pistol for the price, and I am not a rabid Ruger fan.
3) Read Potatohead's thread. Start your own and get some feedback. Also find and watch videos from folks like Todd Jarrett and Jerry Miculek. Establish a safe place to perform some dry-fire routines. Dedicate the time to perfect practice. Find an instructor who will look at what you're doing and break it down to the basics for you. Do NOT go to the range with 300-rounds and a plan to just "shoot it all." That won't help you at all.
4) Practice! Learn to reload if it interests you. You're going to go through lots of ammo and time getting and staying proficient. Steel plates are a great way to make a bucket of ammo disappear without trying.
Lastly, just about any modern manufacture full-size or compact 9mm will last for tens of thousands of rounds, so don't overlook the used case. Of all of the various 9mm semi-autos I've fired since I started shooting, I can't think of one that wouldn't be capable of producing 6" 50' groups, and that includes a couple mini Kel-tecs (P-11, PF-9), a baby Kahr and an old beater Hi Power that didn't have a rear sight. Most people aren't going to put enough ammo through one to "wear it out", so keep your eyes open.
Good luck and good shooting!