Looks like you've made a good start there, more than a good start really.
I'd pick up an
x-acto knife and a bunch of blades. You'll find it a lot more suitable to the sort of fiddly cutting and scraping that you're going to be doing than the leatherman. Honestly I'm not sure what you'd use the leatherman for here.
Testors plastic cement is a great choice. I would never use super glue on
GW plastic-to-plastic assembly. Cement bonds so much better. It doesn't dry that quickly though. So make sure you don't over assemble a model in one go or it will start slowly falling apart from the weight before each join has had a chance to dry. Those three bottles will last a while.
The brush in the jar is fine, just be aware of how much you're bringing up each time. Washing a model in cement won't ruin it but it will make a bit of a mess and might make fine detail a little blobby. Using any other brush would trash the brush.
I generally prefer to scrape away mold lines rather than file them. It's especially easy on plastics and with the aforementioned x-acto knife. I can't recommend removing mold lines enough. It's the single biggest thing you can do starting out to make your paint jobs look better. It's a little time consuming but just throw on some podcasts and get scraping.
The metal file will get clogged up quite easily if you use it on metal models. You might be better off with emery boards. Again though, I don't file that much.
For basing, the static grass should be something you add to your basing routine, not the only thing you use. Something easy that I'd recommend is cement/concrete patch. You should be able to find it in little tubs at Home Depot, it's grainy rather than smooth like you might think. Smear it on your bases and let it dry and you have instant self adhering texture. You can add the static grass in patches _after_ painting.
I'd suggest picking up a cutting board/mat. You could get one of those slightly pricey green/grey cutting mats from an office-supply/art store or go with a cheap plastic cutting board.
Unfortunately you're going to have to work out exactly how you like painting your models before you really know what level of assembly to start painting at.
Personally, I prefer to assemble everything except the gun/gun arm. So nothing is obscuring any significant part of the model. I also prefer to base the model at this stage, prior to undercoating (if I'm doing a straightforward textured base on the model, if it's something fancy I'll paint the model on a stand in plain base and transfer him once he and the base are all done).
Bear in mind, if you go the partial assembly route and want to stick with cement for gluing the finished model together then you will have to carefully scrape away the contact areas to reveal bare plastic, and be very careful not to get cement all over your paint job. On the plus side if you use cement you won't get that foggy mess that superglue can leave on your paint job.
If you have any ideas about the specific colour scheme you're going for people can probably give more detailed advice.