The good news for you is that you will not need to worry about installing individual service packs, as there is one patch that will take you all the way to SP3 Hotfix.
As you are running Windows 7, make sure you are patching and running Trainz as admin. Also, remember to turn off your antivirus software during patching.
It's OK - even I don't always notice older threads.
Most content nowadays is either TRS2004 or TC level content (content uploaded after August is likely to be TC1+2 or TC3 level content) - that is due to support ending for TRS2006 and earlier.
I think more creators are creating it to TS2009/TS2010 level is to take advantage of seasonal changes and things like Speedtrees (TS2010)