In the $500-$600 range would something like this be worthwhile?

Vector Arms AK47 Polymer Fixed Stock US $ 529.95


How would the length of pull on something like this compare to the LOP on an M4 with the stock at the first notch out?

Would there be a better option in this price range for a stamped, 7.62x39 AK? Any compatibility issues to worry about with this model vs. others in terms of changing out sights, stocks, optics, etc?

ET:
Or, would the Arsenal SLR 108 be worth waiting/splurging for? Would the barrel be threaded under that slant break?