My 2 cents.
If possible buy from the same series (you get constant field stop, colour and handling etc)... So I've gone with all Naglers, but you could go Ethos or ES etc...
Forget the barlows ... I have one, never use it ... I don't know a serious observer who uses one with any regularity... More glass, more flare, more weight and alignment issues.
As for what focal lengths to get...
Whatever gives you a 5-6mm exit pupil for your scope. Something around the 1.8-2mm range (this is basically the sweet spot). Something around the 1mm mark, and down the track, perhaps something pushing 0.5mm for very good nights on the planets...
In my 16" F4 I have 31,20,13,9,7 Nag ... The 20 and 13 see the most action, then then 9, followed by the 31 and 7. I also have a 5mm Tak LE for very very good nights - very sharp EP.
This calculator is very helpful -
http://www.davidpaulgreen.com/tec.html