[gregzz4]

If you really want a sump in the garage, a 1' drop and 8' away means you'll need about an 8" drop. More is better. 1/8" per foot is close to code but I prefer to go overkill.
The bottom of your drains need to be, in this case, at least 8" higher than your planned sump. If you can, put your tank on a taller stand.
I was having flow issues with my old setup. In my case, my drain pipes were too small @ 1"
My bulkheads used to be 30" from the floor and now are 40".
I run at least 6' long and use 1-1/2" pipe. Everything has been great for nearly 3 years now.

As for your structural stuff, if you're going to put footings down in the soil, you'll need to dig down at a minimum of 24". 12" is for the footing and another 12" is to clear the frost line. In my opinion it's not worth it. Don't use jacks without a proper concrete footing. It will fail with frost heave if you don't dig it down and pour a proper bed.
Sistering the joists is a good enough plan as you won't get much deflection without them, so I don't see much floor movement once you add material.
Ensure you sister properly. Both sides of each joist need material, and go 100% past your required area.
Use glue and decent screws and leave it alone for a few days to dry.
I'll bet sistering is all you need.

Good luck !!