I'm currently running a 1648 Crestliner mod v, with an older 23 vanguard LT. WANTING not needing a new rig, which would you get first? a new custom hull or a Back water 35/37?

If you're happy with your current setup it really doesn't make much difference which you get first because both will be a large improvement in performance over what you currently run.

If the size hull you are wanting is too large for your current 23, I'd recommend getting the new motor first, then having a hull built for it. If your dimensions are doable for a 23, get the hull built first, then repower it later.... Just my $.02, which I'm sure is worth much less than that.