The website isn’t the best here, but out of that I’d get £70 (I’m in the uk.) I’d rather have it on the market place where it would be a lot more cheaper for everyone and I’d still get something decent out of it too. Plus when i have more time not looking for work, i actually do plan on having more videos and documentation support for this, though I’ve tried to keep it straightforward where possible. Also considering how much of a large mechanic this is, i feel the cost justifies itself considering if you end up using it and selling games.
If you still feel it’s a high price for you, then wait until its up on the marketplace.
Edit: I’ve reduced it by £15($30 cad) as i am currently in a good mood 
If you wanted to change each part of a weapon, say like the barrel, the stock, receiver then all you’d have to do is add more staticmesh/ skeletal mesh components and add the blueprint code to switch those parts using UMG or what other method you’d like.
Just make sure each part is connected to a custom socket on each part of the mesh.
I have it a similar system in the RPG i am making: