I've just done a scan spree, and I've got a few nice planets out of it. The question is, which order should I take them in?
My productions are:
408 minerals
1129 artifacts
1292 research (258 scientists)
I have 2 developing planets too (one artifact, one research, each will add a further 60 to each total)
(And I'm researching from +5s to +8s before upgrading, so I'll be pumping out a lot more then)
My upkeep is 9.6M, and I have 2.25B in the bank (At rank 100 )
Here's the planets:

Massive, ex abundant research and mining (easy take)
Very large, rich mining (mid-difficulty take) (toxic, and I have a purifier)
Large, rich research (x2)
Large, rich mining (x2)
Large, rich mining (but with a stripcore mine that I'd remove and put onto a more abundant planet)
Average, 3x Mega artifact (easy take from an inactive)

What order should I take these in? Which ones should I not take at all? And how heavilly should I defend them?

Massive, ex abundant research and mining (easy take)
Average, 3x Mega artifact (easy take from an inactive)
Large, rich research (x2)
Very large, rich mining (mid-difficulty take) (toxic, and I have a purifier)
Large, rich mining (Stripcore)
Large, rich mining (x2)

Your mineral production is a little behind your others, so jump the mining planets first. Yes, you have a decent stock of cash-on-hand but your gonna need it later...

Only reason I put the 3x mega at (2) is because a multimega planet can get fluxed at any time. If it is a Terra or Desert you can always make it bigger if you haven't used all your Gaia Seeds and Gamma Crystalizers.

The VL Toxic let alone for now. Build up your attack till it goes from being a mid-difficulty take to an easy take. Plus the owner might purify it a little for you in the meantime, but don't let it get too high before you nab it, they could flux it too.

The massive, take it last if at all. 2-way planets are a bit of a waste especially if both resources are only in the abundant range. Massives are nice because you can cram a bunch of stuff on them, but that particular one would need A LOT of work to make it a good planet. You're looking at farming a pile of Labpods for Planet Data Optimizers to get a good production out of it.

The Large research planets, I'll assume those are Gas planets. If they are Gas, they're a dime-a-dozen. If they're not Gas, that size/resource is still a dime-a-dozen. Not really worth an invasion.

As far as defense goes, unless you have at least Planetary Shields and Null Minefields, anything you put on a planet is just for defending against lowbies. 2 best defense and 1 best attack is what I normally put on a standard planet. Good planets get more naturally.

Thanks for the advice.

Just so everyone knows, I don't have either mission planet.

I'll take the stripcore first, then the 3x mega, then another mining, then take it from there.

Just picked up a large, mega rich planet.
Easy take, has a couple of nice structures, but I'd expect a strong counterattack.

Whereabouts should I take it, and is it worth fluxing (I could whack ~150AP out of it with my +5s)

It's worth a shot at taking and keeping at any rate.

I couldn't keep it without fluxing it. The owner's legion would be able to murder me, and take it back EASY. Is it worth going for?

Any large+ mega rich is worth going for if you have a flux handy.