The client (gw.exe) and the game data file (gw.dat) are 2 different things. You need both to be updated to the latest.
The latest up-to-date gw.dat file is typically around 3.8GB. This is mine, I have all campaigns and EotN. I have been everywhere in GW.
What campaigns do you have Soul Assassin? The answer will determine on the amount of updates you will get and also when you purchased the game, since the game on the discs may have been cut with a specific build date / version on them.
I would say that your updates won't be over 1GB. I am willing to bet it will be between 500MB-750MB.
The reason I am saying this is because GW has an excellent compression and distribution method they use to stream the game updates. Once the compressed data is downloaded, it will be decompressed once you enter the game / campaign and travel from outpost to outpost and the data in the .dat is required.
I have done full syncs from scratch, with no CD's and then the most I downloaded was 1-1.2GB of updates, this is 10's of 1000's of files.
It's never too late to start playing. The game plays grind-free in the early levels, and once you hit max level @ lvl20, you can enjoy the game completely. This max leveling process takes at most a week if you play 2-3 hours per day, when playing normally and just following the storyline.
If you have a problem with updating the game, I might be able to put my gw.dat file (3.9GB) online for you to download. Let me know what you decide to do.