Choosing a worldwide stamp catalog was not really a problem for me. I've always maintained a set of Scott catalogs and currently have a 2006 hard copy set, as well as a 2013 set on CD.
These work well for me for a couple reasons.
1 - Since I live in the US this is the catalog of choice by most US-based dealers. So, that makes it much more convenient when going to stamp shows.
2 - I chose the Steiner system of album pages and they are, basically, arranged in Scott catalog order.
But, as I revealed earlier in this thread, my goal is to go slightly beyond what is printed on the Steiner pages and what is listed in the Scott catalog, without going too far into extreme specialization. So, it was clear I was going to need catalogs beyond Scott.
I started acquiring catalogs that covered several countries, and are more "specialized" than Scott, for those countries.
I already had Edifil catalogs, from my Spanish specialization period, that cover Spain and all it's Colonies. Also, A Facit catalog for Scandinavian countries.
Other catalogs I've recently bought:
Maury - for France
Stanley Gibbons - GB and Empire
I'm sure I'll be looking at other catalogs as well, such as Michel, for German area. But I'm not in a hurry.
Inventorying:It was also clear I was gong to need a system to keep track of what I already have, and what I need, at least as far as basic catalog numbers go.
There are apps and software for this purpose that really look comprehensive, but would they work for me? Probably not. Also considered was a spreadsheet system. This would have worked OK, but my intention was to be able to use it on my phone, or wife's Kindle, and I couldn't seem to get it to work very well.
Finally, I made the decision to try and use a VERY simplified list of numbers, using a MS Word doc. This works well for me.
Here are a couple screen captures of one of my Belgium list.
Basically, it's just a continuous list of consecutive numbers, separated by commas, based on Scott of course.
Sets are enclosed by parenthesis and different colors are used to represent different things. All numbers start as black. If I have a stamp in used condition, the color changes to blue. An unused stamps (MNH, hinged, or whatever) is turned
green. Damaged stamps, that need upgrading, are in
red.

Air Mail, Semi-Post, and other back of the book stamps have their own sections. But I didn't like typing in the prefix before each number, so each section would be labeled such as "Airmail C#'s". That way I could just copy and paste the actual numbers from a master file.
