In terms of specialization, that's up to you. The old saying "whatever floats your boat" applies here. If you narrow your usual suspects down to one category, say WWII tanks (or even further down to German tanks of WWII) your greatest price will be the opportunity costs of missing out on an entirely different subject matter, say modern Western jet fighters, and their newest state-of-the-art kits.
I know some modellers who simply want to build the latest, most highly regarded models regardless of genre. This has the advantage of generally rewarding them with easy to build, great kits ootb (out of the box). Doesn't matter if it's a TIE fighter or a gebirgsjaeger, these individuals just want the latest kits creating buzz. Nothing wrong with that if that's what motivates you.
Others take a different route. These are the specialists. The ones motivated by one type of kit, and/or one historical period. I fall into this class because all I build is WWII armor (well, I've occasionally cleansed my palatte with a WWII winged thingy but that's our secret). My reason: I'm a WWII history buff. Have been all my life. And to me, tanks just have more visual appeal than other WWII subjects. Not much more to it than that.
The primary advantage of specialization is knowledge: both of the internalized kind (memory from reading, studying, etc.) and the externally accessible kind (books, videos, internet links, etc.). When you narrow your focus you make learning minutae about your subject much easier (and cheaper!). To me, this makes building accurate models easier (then again this rests on the assumption that accuracy is important; not all modellers agree, nor should they--build for yourself, not others.).
Another consideration is that different subjects may require different modelling skills. For example, the airbrush skills required of a car modeller (think smooth and shiny) and those required of an armor builder (think camo and fading) are vastly different. Weathering and finishing are other areas that differ widely. Switching back and forth between disparate genres may slow down your ability to master the different skills required by each genre. Then again, spending time improving that widened skill set may be personally rewarding to you.
One concern I will put to rest, just in case you have it in the back of your head: Don't worry about there being too few kits to pique your interest in a specific genre of modelling. We live in a great time for builders of almost any genre (except for allied WWII halftrack nuts of course, but don't get the wrong people started on that!). Your challenge will most likely be winnowing your chosen kits regardless of genre, not finding enough to keep you busy.
One last thing: Welcome to Armorama!
