If you want to do well in this game you will need to spend money. If you are not spending money you will eventually reach a plateau of power and abilities, and advancing will slow down to a crawl. You should make a conscious decision on whether or not you want to spend money on this game. If you do decide to spend money, you should set yourself a budget - decide how much you will allow yourself to spend each day or week. AMS is constantly tempting you with deals, and many things you simply cannot get unless you buy them. Building and research become impossible unless you spend money. Some officers cannot be attained unless you spend money.
The first of these officers is Wendy. Wendy is one of the more powerful officers in the game for Special Operations and Counterattack. AMS tempts you with Wendy with tons of notifications, and even allows you to try her out to show you how powerful she can be. In the initial stages of Special Operations and Counterattack she can be very powerful, and she can be assigned to a position in Officer Assignment (more on that later), however as you advance in level she's not that important (with the exception of that officer assignment). Further, just buying her isn't the end of the cost for her - you will want to improve her Skill Level, and doing so will likely require you to spend more money to do so.
It's a slippery slope, and AMS intentionally sets it up that way. They figure that if you'll spend $5 for a deal today, you might feel better about spending $10 next week. They offer tons of deals specifically designed to rope you in. There are players who spend THOUSANDS of dollars on this game. It becomes addicting, and as you see your friends and adversaries improve in power, while you fall behind, you become tempted to spend money to catch up to them. Your best bet - for your own personal sanity and financial health - is to make a conscious decision and stick to it. If your checking account allows you to spend $100 on a game every week to have fun, go for it. If your family doesn't mind you spending $5 a day on entertainment, have at it. But be careful about letting yourself get pulled in to increasing that amount. It's easy to do. Eventually that piper will need to be paid.
So, back to Wendy - she's available for purchase in Special Operations, and if a player is going to spend money, she's likely the first purchase the player will make. Make a decision. Think about it now - will you be spending money on this game? If you decide yes, you may as well purchase Wendy. If you decide that no, you will not be spending money on this game, then you need to get used to the fact that you will receive constant temptations from AMS to spend money, and you will need to get into the habit of ignoring those temptations.
Something else to realize about spending money: Almost EVERYTHING is available on sale at some point. EVERYTHING! If there's a new plane, eventually it will go on sale. New officers are available all the time - they almost always go on sale. It might take a few months, but eventually it will be available at a discount of 90% or more. Frequently you can buy one thing, and have that purchase go towards meeting the requirement for an Event that gives you even more free stuff. Be wise about your spending. It's not always important to have that shiny new officer on the day it comes out. If you're patient, you will almost always be rewarded by being able to purchase it at a fraction of its' initial cost.
That goes for items and resources as well. There are deals upon deals that can be found that will allow you to purchase what you need at a huge discount. Sure, you can buy one Advanced Teleport at 2,000 gold (which roughly equates to $20), but sometimes a pack will be made available that allows you to purchase 300 Teleports for $20, which is a fraction of their normal cost. You just have to be patient. Ask your fellow players if an item will be going on sale, and find out how frequently it happens. Hopefully much of that information should be available on this wiki.
There are oftentimes deals available on gold too. The base rate with no discount for purchasing gold is $1 US for every 100 gold. However there are lots of ways to get gold for less money.
As mentioned before, frequently making one purchase can advance your progress in an event that gives you other things either for free or at huge discount. Look for these! Again, these should be pointed out on this site. If you plan on spending money - spend it wisely!
Smart initial purchases:
Wendy - can get her with a $5 purchase.
Possibly Gent Code Glory (likely to get Soldier-X) $20