AIR TANKS
Here at High Pressure Airsoft prefer Ninja Brand HPA tanks due to their proven performance in the field. There is a lot of personal preference that goes into "Which tank should I buy?", but if you have the money and a local field or shop that can fill to 4500psi go with the 90cu 4500 psi carbon fiber tank. If you want to save some money and weight isn't an issue for you then select the Aluminum 60cu 3000psi tank. Another thing to consider is if you will be purchasing a scuba tank for field refills then you can't fill beyond 3000psi anyway so a 3000psi tank would be a great option. There are many options on the market from less expensive to more expensive and all will work, but always consider how you will be refilling and how much weight you want to carry with you.
If you are not purchasing your HPA system in a kit that includes all neccessary elements then just know you'll need and air rig such as the SLP or LPR/FireBase rigs. Both rigs are great, but there are a couple things to know about each. The SLP rig featured top left is only compatible with SLP air tanks of 3000psi or less. It is a larger rig than the LPR so if weight and space are of concern, consider the LPR. The nice thing about this rig is that it's quickly adjustable, however if you play tournaments you'll have to locate a tournament lock out device to keep you from the temptation of increasing pressure after chronoing in.
The other option is the NINJA LPR/FIREBASE rigs which are the preferred rigs on the field today. Extremely reliable, smaller and more streamlined design and built in tournament locks are just a few reasons why it's the preferred regulator. It isn't as quickly adjustable as it does require an allen wrench for adjustment, but you shouldn't need to adjust in the field anyway. These regulators do cost a bit more, but really are worth the investment. The Firebase is the most expensive with the Ninja LPR being a more wallet friendly direct clone, you can't go wrong either way.
AIR RIGS
The last element in your Air Tank/Rig setup is the hose or line. If you can, get yourself a widebore, 36"-42" line with dual QD's or Quick Disconnects. Don't get too caught up in the lines though as all will work as long as they have the correct fittings for your HPA engine of choice and air regulator of choice.