Depends on a whole lot of factors. The first and most important thing is that your personal safety equipment has to be legal for where you want to run. So if the rules say, "No red fire suits" then you would be really unhappy if you showed up in a red suit--even if that suit was just as safe as a blue one.
So the very first step is to find the rulebooks for all of the groups that you might run with. If you can't get ahold of rulebooks, then get ahold of the "safety steward" or the club member in charge of safety. Find out what the minimum requirements are, and then use those to rule out things that aren't acceptable. (For instance, some places will be fine with motorcycle helmets, others will require auto racing helmets specifically.)
There are a number of different systems for rating safety equipment. The most popular one for helmets here in the US is the Snell Foundation; they typically have standards that are updated every 5 years. They offer several types of certification, so usually the Snell rating is a letter and number combination. M2005, for instance, denotes a helmet approved for Motorcycle use in the 2005 update. SA2015 would be the most recent "Special Application" (auto racing) helmet. The FIA might also have ratings for helmets, but I'm not sure.
Fire suits will often have an "SFI" rating, which I think is in minutes of exposure before you get 3rd-degree burns? Something like that. FIA also has an approval system, which I think is a yes/no, and an expiration date.
There are likewise approvals for seats, for belts, and so on. In recent years, the requirements for belts have been updated to say that they can expire; belts that are too old are not considered safe until the webbing is replaced.
In general, the wider the belt is the better it distributes impact forces across your body. For the lap and shoulder belts, you usually want 3" wide webbing. You also want to install things such that the belt stays in position, which means the angles relative to your body's seated position have to be in a specific range, and you must have one or more sub straps to make sure the lap belt stays low where it should be.
There are several head-restraint systems available. Not all are accepted by all sanctioning bodies. Find out what is OK by the places you want to run. HANS is one brand, and it assumes that you have properly-attached shoulder harnesses holding the collar in place. There is another brand that separately straps the collar to your body, and others that limit your head's motion in different ways.
A lot of the differences come down to your own preferences. What fits you well, what is your budget, what provides a level of protection you think you can live with, and so on. Especially when it comes to helmets and seats, there is no substitute for trying the actual thing you are buying. Both seats and helmets need to be snug enough to stay in place (or keep you in place), but not tight enough to be uncomfortable over the course of 1/2 hour to several hours in a row.
Lots of places for mail-order. Simpson typically makes pretty good stuff. But there are others.
"Mango Bango", 1990 CRX Si -- Sold on 10/01/11, with over 227,000 miles.
VTEC is Honda's "select a cam profile on the fly" system; VTECH is a crappy phone.