[Greylocks]

This follows from folks saying I'm too negative. Really long post...

What follows applies to (almost) any AEG out there. There are so many brands that I wont bother mentioning any; just find parts that have proper specs, any of the Retailers will have them in stock.

Smart; start by asking for HELP, specially the first time you will take that gun apart. Also do your research because many upgrades require other important changes that are not always obvious.

So, what is smart? Smart are changes that will improve the function of the gun without causing damage to other parts, or people.

Example; Marui really should sell their guns with metal bushings pre-installed. It's one of the first parts you must add with any spring upgrade. Now other companies have guns with metal bushings from the factory.

Smart spring choice; you want a power level that only requires changing the bushings. If it starts to chew up your gears or your piston, the costs escalate. (Edit: the quality of some springs has changed since I wrote this, so pick a spring that will give you between 320 and 360FPS at the most, from whatever brand is recommended at the moment). A spring that gives that FPS will work well, and will be safe for the gun and everyone.

Other smart suggestions; the best battery to match the upgrade. Most guns are designed for 8.4 volts, so to avoid electrical problems stick with that. But the MAH rating is often not enough. Do your best to save cash for batteries. A pair makes sense. Dont scrimp on the cost of the Charger. Badly charged/discharged batteries are useless and costly. Investing in a top of the line charger pays for itself within a year.

Good ammo; dont buy crap. Enough said.

Basic maintenance; buy the right tools, oils, a good protective case or bag. Look for something discrete if possible.

Not so smart upgrades (or stuff that can certainly wait);

Scopes of all kinds. Airsoft guns rarely have the range to make those worthwhile. They also dont tend to have the accuracy for which a scope becomes truly useful. Red-dot sights are a reasonable choice, but they can also wait.

Cosmetics; a pretty gun wont do much for you, really. That can wait. Some changes in that area can cause more damage than it's worth. Most airsoft parts are plastic or alloys that dont do well with some paints or chemicals. What happens next is that you suddenly find that getting spare parts is a real pain in the behind.

Barrel changes; fairly easy to do if the barrel remains the same lenght as the original. Is it really necessary? That's debatable. Many players have excellent results with stock barrels, some swear by the tightbore. My criteria here is simple. Is the expense worth the gain? So far for all the guns I owned, the answer is no.

If you change to a longer barrel, do your homework once more. Airsoft guns, when you step away from the looks, are nothing more than an electrical/mechanical contraption that must follow the laws of physics.

The longer the barrel, the more pressure is needed. Mechanically, this becomes a good puzzle to match several parts. Expensive too.

Accuracy; this is a tough one. Airsoft are creatively modified smoothbore, and the ballistics are more akin to a musket or shotgun than a rifle. The most you can expect is about 150 to 200 feet. The solutions for airsoft are tighter bores and the backspin from the hopup system. If you adjust the hopup properly, even a stock gun or one with mild upgrades will make you a really happy player.

Speed; not enough is not good, too much is bad. That also includes motor speed. High rates of fire are fun, but really they are not that important. The stock motor is just fine. You also diminish the wear and tear. Above 400fps you get some strange problems that have a lot to do with basic physics. A light and round projectile just cant stay stable if pushed too fast. Fact of life. Safety has been discussed to death, and many places have limits anyway. There's a good reason why intermediate powered springs are strongly suggested.

Other accessories; metal bodies, silencers, bipods, lights, lasers. Metal bodies are not necessary at all. Nice to have later, but not needed. Silencers look cool, change a bit of the sound, but that's about it. Many folks use them to hide longer barrels.
Bipods rattle too much, add weight, and are not used that often. All I've seen them used for was to display the gun on a table.
Lights; look cool, may be useful if you do a lot of gaming in CQB, indoors or at night, but that's about it. They (the good ones) are expensive and not really useful.
Lasers; useless in daylight, point back to you at dark, banned in many places due to the potential eye damage. Yes, police and military use them, but when the risk is to get shot for real, then a laser hardly matters.

Accessories to buy; spare ammo, spare magazines, spare parts, spare batteries, good tools, cleaning supplies.

As a last word, please, for your sake, swallow your pride and ask for help. I know a lot of folks who will help you for free, no charge, because it's fun.