How well does the ammo shoot out of your rifle?
"...What else am I going to "need" to attend this shoot and have a good time?..." Ear and eye protection. Likely more than one mag, but eyes and ears is far more important. Take some kind of water container too. Ranges tend to be dry and dusty places. If you have grass related allergies, take your meds with you.
"...Is my rifle adequate..." Close enough. It's also what you have. Standard M1A's aren't target rifles, but it'll do if your ammo shoots well. What you'll see will be gussied up, at great expense, match rifles. In any case, there's a timing learning curve. 60 seconds for 10 rounds at 200 sounds like lots of time and it is, but you have to do it a few times to get used to the idea.
Read this.
http://www.nrahq.org/compete/highpower.asp
However, just go and don't worry about placing. You'll have fun. Competitive shooting is just like that. The other shooters will fall all over themselves to help you. Including letting you shoot their rifles and suggesting improvements. Bedding and doing the trigger on a Standard is a relatively easy, inexpensive, thing. Putting on a match grade barrel, not so much.
I've found any kind of match shooting to be so much fun that I didn't care if I placed or not. The people are fabulous and they really will go out of their way to help a new shooter. Don't worry about the occasional rectal orifice either. They're everywhere.
Oh, and volunteer to help with anything that needs doing too. You'll meet more people and, despite appearances, volunteer help is appreciated. Even if it's just moving garbage. New guys that help tend to get more help than new guys that expect to be helped.