If you're able to run EaW/ICW, you shouldn't have any real problems running Sins/Ascendancy. You'd just have to do it on low detail, since that's an integrated graphics card which aren't especially great.
As far as where you're buying it, you shouldn't need to buy it from anywhere that needs to ship anything. As far as I'm aware, the game is only available via digital download. It may be $40 on Steam now, but it goes on sale pretty often. At least once a month it seems like, for prices between 7.99 and 20.00. The Steam summer sale was actually the second time it was on sale in June.
I run integrated graphics, I can run everything on Very High. I can max settings on EaW too if I go through the bullshit with certain cards not being allowed to turn certain things up past a certain point, which I think I already did. Sin's only real issue is the fact that it's single core and late game lag gets bad no matter what your machine specs are.
Corey is right though, DO NOT BUY IT AT FULL PRICE. It went on sale once at the end of may and then multiple times in June. I grabbed the game and all DLC for like $10 total a while ago. Buy it digital, also.
If you want to be kept up to date on sales just google "Sins of a solar empire sales" every so often and you should get something. Otherwise, check /r/gamedeals on reddit every so often.
You should be able to run the mod fine if Ascendancy is going to be anything similar in relation to optimization to Sins of a Galactic Empire or Sins of the Prophets. Even so, Sins isn't a hard game to run at all.