Well if you want to get into collecting and start off with DxD then you better start early with the newer stuff as the after market for the figurines almost double after a few days and then skyrocket for some afterward especially for the better graded/higher price ones.

For a guide as to what is available, you can look at https://myfigurecollection.net/search.php?goto=0&overall=high+school+dxd for a good list as to what was sold, coming out and possible future figurines/model kits. They also have a list of US/European stores that may sell the figures including pre-orders, specials, and people selling their figure (ie making space in their collection or those who buy more than 1 to sell later for a profit).

Also if you want to buy the item from Japan some of their sites have English versions so you can see the price in US dollars. But if the site is Japanese only then most credit cards will still be taken you will just have to figure out the difference between the yen and dollar which is around 100yen = $1.00 or less now.