About Unsaleable, you are right, they can be expensive. If I remember, they ship from Austria... could there be a link to Lomography? Lomo is also quite expensive. But if you want to get SX-70 Blend film, they're the only place to go.
As for sensitivity, SX-70 Timezero film is rated at around ISO150 (from memory) while 600 film is rated at ISO600 (duh).
SX-70 blend is essentially 600 film with a built-in neutral filter that has a slight color shift to make it look like the original Time Zero.
This doesn't need a ND filter on the camera.
On the other hand, the ND filter is a one time buy.
But for the best mod in town, consider a capacitor replacement. Very deep surgery but it fixes 2 problems with one step.
- It corrects the exposure meter so 600 film can be used
- Capacitors tend to dry up with age and malfunction, this fixes it for good
So there you have it... hope it helps.