What kind of budget? I use GW cases, but that's out of sheer momentum. I already own them, and they serve the purpose. If I were to start over I'd probably use
Battlefoam or Sabol, likely Battlefoam. I'm using some Battlefoam trays that fit GW cases for my Guard, and love them.
As for what fits, I have about 25k of Guard in 7 GW sized trays (trays for battlefoam's own bags are slightly larger), all in a large GW case. My nascent Ork army has 3k of infantry plus a dozen tanks or so in a single GW tray. Remember you can fit 4 stands in a 1" x 1.5" slot in inch thick foam.
Interestingly, you can fit a lot more models in a GW case with Battlefoam than GW trays, due to battlefoam slots being denser ( 5 x 11 instead of 4 x 9 for GW), and slightly thinner (you can fit four Battlefoam trays in a GW case, but only three GW trays). This doubles your storage space (220 slots versus 108 in a small GW case).
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SG
Ghost's Paint Blog, where everything goes that isn't something else.