Get a good owner's/operator's manual from Fulton Armory or on amazon.  Don't get a manual that is for collectors or such right now.

Read the manual, but good, and buy any special tools you might need/want.  At minimum, a coated cleaning rod of correct length, muzzle guide for same, bolt roller lubing tool, and bolt assy/dis-assy tool.  Get a GI buttstock cleaning kit for your rifle (not a Garand), wipe it with oil, and stash it in the buttstock whehre it can never be lost, and will always be available.  The added recoil-damping mass also changes the center of gravity a bit, and makes the rifle feel a bit more "lively".

Take the rifle apart, clean every bit of it, lube it right, and go shoot it.  Take an Appleseed course, they will show you how to do it right.  also see my thread in .22 RF forum here on inexpensive 10-22 Training rifles, which you can use to save $$$$ while training to shoot your M1A.  I take my 10-22 out 3X more than my M1A, and all the training I do is directly transferable.

DO NOT modify your new rifle (and mebbe void warranty) until you have put several hundred rounds through it to confirm reliability, unless an obvious problem crops up sooner.  Test each mag, and mark them individually so as to be able to ID problem mags.  Buy good quality web gear to carry your mags, water, and so forth.  GI loadout was 5 mags, and maybe a bandolier or two.  If you find it worthwhile, have SA do a trigger job on your trigger group.  Most folks start there with mods, and having them do it will not void your warranty, either.  Probably the most cost-effective mod you can do.

Spare parts to obtain, either NOS GI, or well-made civvy, are firing pin, hammer and trigger pins, all springs, extractor, and ejector.  Buy a GI synthetic stock to save your new wooden unit.  Syn stocks can be easily painted, and do not warp due to humidity, unlike wood. Many say the tighter fit of GI syn stocks enhances accuracy.

Using decent (not new, fresh) GI milsurp ammo, your rifle should group within 4 MOA, that is to say, if you are a good shooter, the rifle should group within either an inch at 25M, or 4" at 100 yds.  You can get great ammo for your rifle, but it is expensive, even if you re-load.  See my thread on the 10-22 mentioned above, and start saving all your boxer-primed (reloadable) brass.

Some Burris pistol scopes are seemingly made for your rifle, and give reticular hold-overs out to 500M.  Save up for such, and in the meantime calibrate your iron sights at 25M, setting the index on elevation to "2" when you are right-on.  Confirm zero at actual range ASAP.

Above all, do not rush out and try to expensively modify your rifle.  Spend time and effort in learning how to use it right, and by the time you decide some mods might be in order, you will have had the chance to save up for them, and will have also had the benefit of time to ask other owners their opinions.  You will have plenty to do if you follow my suggestions.  As the owner of a SM M1A, they are all made from experience.

Having been to an Appleseed, let me assure you that it will be worth your while.  I drove about that distance, and rented a cheap motel room to boot.  Worth every penny I spent, and more.

I practiced beforehand, mostly in the snow, and I made Rifleman on the first try.  Some folks didn't, but that is not unusual, either.

Get "Fred's Guide to Becoming a Rifleman", some targets, and a very good copy of the GI shooter's coat in a package deal from fredsm14stocks.com––Click on "Latest" on the left of the page and scroll down a ways..  The Guide contains a LOT of info on the M1A rifle, and a ton of shooting tips.  I bought the package, and it is a great deal if you read the Guide and PRACTICE.  The shooting coat comes in mighty handy, and you can take that to the bank, too.

Unlike the stocks, the Guide/target/coat combo does not require much "hand-select" work, and Fred says on his Site they ship a lot faster than the stocks, which he also admits take some time to ship.

Stick to the basics, and you'll be fine.  I think you have the right attitude; buying slick stocks and so forth does very little (if anything at all) for you except take money from things you really need (mentioned above), and practice ammo.

It makes more sense to me to shoot what you have up to its' and your limit, and mebbe then modifying it.  It will take you some time and practice to get to that point, and by the time you get there, you will have a much better idea of what you need than you do now.

We used to have a member here who had to have spent a fortune on gizmos for the M1A, and I'm not kidding about the money, either.

That's fine for him, mebbe, but he was mighty shy of going to invites to shoot the things upon which he had lavished so much cash.  Always some excuse, and many invites.  Wonder why.....

The 25M zeroing trick will put your point of impact very close at 200M.  Dial-in at actual range for fine-tuning, and then adjust elevation wheel to "2".
Your "battle Zero" on a Metric rear sight is 250M, and on an Inch rear sight, 275 yds.  Therefore, you will have to click up to reach battle zero.

All of this presumes fresh, GI/milspec ammo with a muzzle velocity of 2750 fps out of a standard, 22-inch barrel.  In this application, a general rule of thumb is that the bullet will lose about 100fps for each inch of reduction in barrel length, so your 18" bbl will be starting out with a muzzle velocity of approx 2350 fps.  Note that the rear sight is calibrated to suit the standard-length barrel, not the shorter one.  At short-ranges, the difference in ballistic path is very slight, but at very long ranges, the differential becomes progressively more pronounced.

You can almost certainly adjust your rear sight calibration to fudge this difference at short ranges so that the rear sight elevation markings will be spot-on at short ranges, and then begin to differ objectionably at longer ranges.  You will have to refine these numbers before plugging the figures into a ballistics program.