OK Quadra synth and JV80 are a bit dated now for sure. If you had said Kurzweil then that would be a different story!
For pop music you need a decent array of bread and butter core sounds to start with. Drums, (percussion, electronic kits loops etc) basses, guitars, pianos (acoustic and electric) , organs, strings and assortment of keyboard sounds etc. I would be looking for the best instrument that is going to give you these at least straight up and you can add to later.
Native Komplete Ultimate while very impressive might be too much. Kontakt on the other hand might be perfect. Alchemy is a bit of a weird animal. Capable of doing a lot but it might not be a good pop machine though.
Some instruments like Omnisphere have millions of amazing pads and atmospheres but are they going to really help you in a pop situation. Sometimes these types of synths sound so good (on their own) that you cant use them! Does it cover the real core basics well. (I don't own it so Omni people should elaborate)
If you were doing complex ambient music or cinematic trailers then some of the more interesting machines are going to cut it.
Sounds like it needs to sample based at least so you can get the core sounds down. Kontakt sounds ideal but it is $400 which is a lot. Or Komplete 9 at $500 rather than ultimate. For the same money as Omnisphere it seems like a better deal. You still get a ton of instruments. Or you could just start with Kontakt (which is huge) and upgrade to Komplete later. (for $99)
Is Zebra going to give you the core sounds you need? Maybe not. Fabulous instrument but you have to be careful as to what some say about it. They might already have all the core basics down with other VST's and Zebra would then be a wonderful addition.
Do you already own any VST's that are giving you great core pop sounds. If you do then you need something to add to that but if not you really need to think about establishing the meat and potatoes stuff first. The Emulator X2 is another good option as well. I started with a hardware Emulator and still have a high powered rack with over 70G of sound library and there is still nothing it cannot do!
Another option is to buy a series of smaller VST's to cover all the bases you need eg a great piano VST, organ VST, drum VST etc but in the end that might end up costing you more. Komplete starts looking pretty good then.
Are you opposed to buying another hardware synth. The Kurzweil PC3K6 is a killer instrument and with the Kore 64 expansion installed has thousands of patches and the thing is it would eat the pop music thing for breakfast. But it can do all the Omni stuff too. But I do agree hardware is expensive and soft synths are much cheaper and still sound amazing.
Anyway just some ideas to throw around.