Pete
Welcome to IIS!!
Alan's advice is the one I would have posted. Particularly about holding off and getting the best from what you have. You will find that the 30mm will be used the most, probably 80-90% of the time with the 15mm most of the rest. The 9mm will mostly stay in your tackle case. Still get it out especially if the night is really good and steady.
The various EPs are a serious question and contention among observers. I am unashamedly a Televue fan and I believe for a serious observer, getting top notch EPs is absolutely a necessity.
Having said that it takes knowledge and experience to fully understand how and why the various EPs vary and the benefits they bring. Take your time, try to get to observing session and try out some various EPs and decide which way to go.
Cheers
Malcolm