Alright, there are two things to keep in mind when purchasing equipment. The first is the rating of your Wealth trait. This determines how many C-bills you have to spend on your starting gear. If you have Wealth of 0, you start out with 1000 c-bills to spend. Go negative on the Wealth trait and you're looking at 100 c-bills. If you're really lucky, you can get up to 2 million c-bills to spend on gear. I'm not quite sure what you'd do with 2 million c-bills, but fortunately any c-bills that you don't spend at the end of character creation get tucked away in your wallet for use once the game starts.
The other trait that determines what you can purchase at the end of the character creation system is your Equipped Trait. This determines what's the highest Tech Rating, Availability Rating and Legality Rating of gear you can purchase.
For instance, this character has wound up with 200 XP towards the Equipped Trait, which counts as a +2. Checking the chart in the Trait section, we see that the the Tech Rating of any equipment we want to purchase is limited to D or lower (A-C), the Availability of any item we want to purchase must be C or lower, and the Legality of any item we want to buy must also be rated C or lower. In the end, our rating is D/C/C.
There are two caveats to this. First, if you're a Clanner, no matter what the chart says, you get to start out with a Tech Rating that's one higher than what the chart says. So in this case, if this was a Clan Nova Cat character, our rating would be E/C/C instead of D/C/C. Likewise, if you're a Periphery brat, you get -1 to your tech rating, or in our case, you would wind up with a C/C/C rating.
The second caveat applies to the Availability and Legality rating. Some pieces of equipment are found to be the purview of a single faction, and found rarely outside of that faction. Such equipment has a Affiliation code. If that affiliation code doesn't match your character's affiliation, then you have to increase the gear's Availability Rating and Legality Rating by 1 each.
For instance, lets look at the Nambu Auto-Pistol. The Equipment rating according to ATOW is "C/B-C-C/C" The first C to the left is the the Tech Rating of the equipment. The Middle three characters, separated by dashes are the Availability rating, one for each main Battletech Era, Star League, Succession Wars and Clan Invasion (IO provides the Availability Code for later Eras). So comparing that to our D/C/C, we can purchase it. The Nambu does have an DC Affiliation tag, but since we match it, the ratings aren't modified. If we were playing a Capellan Character, the Availability and Legality ratings would be modified, making the Nambu "C/C-D-D/D" Since both the Availability and the Legality ratings would be higher than our Equipped trait, if we were a Capellan character, we wouldn't be able to purchase it.
So. We have 5000 C-bills to spend, and can purchase equipment with ratings up to D/C/C.
2xKnives
2xThrowing Knives
Nambu Auto-Pistol w/3 reloads
Laser Sight
Holster
DC Uniform Kit (No Helmet)
MechWarrior Kit (Regular)
Swimsuit
Military Communicator
Video Camera
Noteputer
Sunglasses
5xMicro Power Packs
5xPower Packs
Slug-Thrower Kit
Advanced Field Kit
Medical Kit
Flashbang ZZ10000 Motorcycle
Katana and Wakizashi
So. 3799 C-bills spent. Probably the first noticeable bit is that I'm missing the helmet from the DC Standard Armor Kit. That's because the Legality of the Helmet is D, which is too high for me to afford (more on that later). People that are following along particularly closely might note that the Katana's Clan Invasion Availability code is D, but I checked IO, and that has it listed as C, so I went with that, and assume there's some future errata someplace. The Flashbang motorcycle lets the character get around when not on duty, and is the main reason the character has the "Drive/Ground Vehicles" skill. With a little over 1200 C-bills left over, there's plenty of cash to pick up whatever you might want, within reason.
Why I'm not worried about the missing helmet. There is an optional rule that states the GM can allow a person's faction to issue them gear. So it belongs to the faction and not the character, and it allows you to ignore the cost of the item, and it allows you to acquire 2 pieces of gear with ratings up to E/D/D (or F/D/D for Clanners). These have to be approved by the GM and should be integral to what your character does. For instance, this character could try to convince the GM to let them acquire the Helmet via the Issued Gear rule.
So there you have it. Perhaps a bit more verbose than I should have been, but people seem to get confused on the ratings. So if you have any questions, let me know.