Like you I'm new to Airsoft wargaming and I shall be looking for a decent sidearm myself. I have been looking and looking but to be honest I have no idea what the hell it is I'm looking at! I have some knowledge of firearms manufacturers but this is totally unrelated to Airsoft manufacturers so the research you have obviously done will save me a great deal of time and the codes will help me to decide, which I prefer the look of and which is easiest for me to shoot.
Firstly, sorry for the long post.
In my opinion, you should look at what makes airsoft appeal to you. To me, "looking cool" plays a big part of my fun in airsoft, so I would shorthand a list of sidearms you think "look cool" and then look up which manufacturers make it for airsoft. Assuming you are looking for pistols, good brands to look out for are any brands that base their systems on the Tokyo Marui gas blowback system (the majority of gas blowback pistols are based on the TM GBB), or even the KSC System, although these are getting a bit less popular now. Compared to 10+ years ago, there are so many choices for sidearms it doesn't really matter which you choose. But one thing you have to be sure of is the availability of aftermarket parts for when you need replacements.
TLDR; Pick a pistol that either runs on a reliable GBB system, eg. Tokyo Marui, KWA, System 7 etc. KJ (TM, WE are based on the TM system, although there are other brands use it too. Maybe Umarex, but not 100% sure), The KWA System is also reputable and widely used especially in the US. Both TM, KWA, KJ and Umarex systems have great aftermarket support for spare parts.
Remember that in airsoft, manufacturing tolerances are much looser than the real steel world and you should expect things to break after use. Some brands/systems offer more longevity than others, but any system you choose will eventually fail to some extent (some earlier than others). The most popular systems will be the Glock and 1911 platforms and those will have a plethora of aftermarket parts and support available. However, this is not to say other systems won't have any at all, you will just have less choice for parts for eg. the USP or M9.
Look up the Novritsch SSP1 and see how it's system operates. (Novritsch has a second YouTube channel explaining it in detail.)
Edit: Keep in mind, that even if a system is based on the TM GBB system, it may not be 100% compatible with it. For example, the WE Glock 18c is based on the TM system, and shares certain parts with it, but the slide would not be compatible with TM slides. I personally own a WE glock and use TM upgrade parts in it. The slide would be the last thing I would expect to break, so the incompatibility with TM slides doesn't matter much to me.
