I'd still advise patience and continuing to look for a gently used lefty. Even a no frills build is going to cost you more than a used Series I or II.
Yes, used lefties are relatively rare, but they do appear. You will get A LOT more for your money and if you ever decide to upgrade (there is no question that you will, especially if you visit the mother ship or go to a gathering of Alembic players) you will get back what you paid for it.
A few quotes regarding this come to mind here:
All things come round to him, who will but wait
(Longfellow, Henry Wadsworth, 1863). The Student's Tale. AND
Serene, I fold my hands and wait,
Nor care for wind, nor tide, nor sea;
I rave no more 'gainst time or fate,
For lo! my own shall come to me
Burroughs, J. (1876), Waiting.
In addition (because its just how life all too often works)...as soon as you put down the $$$ for a build, you just know that the perfect lefty for you is going to appear for a very reasonable price. I say this from experience (twice over).