Before answering your questions I’d just like to say that you are off to a great start. The figure in the main picture has a good movement to it and feels like a natural pose.

1. If you don’t already keep a bits box. This should be where you keep spare parts from sprues or that you have cut off any models you have been kit bashing. I also chuck in things that might be useful that would otherwise end up in the bin, wire, mesh, tubes, scraps of plastic packaging that can be repurposed, just whatever you think might be useful.

2. I use unscented Dettol. Leave plastic or metal in it overnight and use some fresh Dettol and a toothbrush to clean them up. Just be sure to put plenty of paper down underneath as the paint gunk dries and can pretty much only be cleaned up by scrubbing it with more Dettol.

3. Looking at your other comments I would also recommend the Howling Griffons, but if you have a colour scheme that you think would look cool just use that and make yourself a nice successor chapter. Personally one of the things I have always enjoyed about 40k is being able to make up your own lore.

One last piece of advice if you plan to do more sculpting with putty. Try getting your hands on some greenstuff, it is my modelling putty of choice and gives the best finished product in my opinion.