Re: I need sustain, and I'm prepared to buy anything.
Honestly, I'm somewhat convinced its mainly down to the necks being so thin, and all my guitars being bolt on. I've found I tend to just get a more stable, better sounding guitar out of a slightly thicker neck whenever I've tried them in shops.
Solid Body wise, currently I have an RG550EX, S7320, and an S2170FW which is actually the worst of the lot in terms of both attack and sustain - it's just always felt like there's a blanket over it, in fact that was the major factor in that guitar getting an Evo2 set. This is despite having had the pocket routed deeper by about 5mm to correct the gap between the fretboard overhang and the body. The S7320 sustains the best, but that's the only one with a neck shim.
Action on all of them has been adjusted many times, and tbh I've always favoured a medium action - too low is.... well, it's too low. I dig in a lot when I play so I need a guitar I can make respond. I had an old strat copy that was set up even a little higher, for the big SRV sound, so I've no problem with a little action, as long as it's not crazy. A Jazz 3 fits perfectly between the high E and the 12th fret of my S7320, so that's 1.38mm action on the high E. Low E wise, probably about 1.7/1.8mm.