i'm a bit late to the party, but i'd love to see some improvement there as well. and my two ideas were also 1) provide filters for weapon type and/or 2) group weapons from different sources together in some way.
sidenote: you guys sure make some great looking mockups!
also, it would be nice to be able to use the shift-click functionality (which puts the last slotted weapon into the currently selected mount) in a way that combines the various possible sources. at the moment, it doesn't allow for quickly slotting weapons from a market after one from the inventory or storage. which makes sense to avoid potentially unexpected expenses, but also makes it less useful for when i either already know how many of those weapons it would have to pull from a market, or simply don't care about the costs (which happens rather early on with cheap weapons like small-mount PD).
if the potential for unexpected expenses is too important to ignore, ctrl-click (which, afaik, doesn't currently have any effect) could be used as source-independent alternative to shift-click, using any weapons available from inventory > storage > military market > open market > black market.