There is a forum for maples and lots of info re: Japanese Maples there. Good luck.

We really need a little more info to give you good advice about soil amendments. What type of soil were you planning on using? How deep will the bed be above ground level? How large will the bed be area wise? If your bed will be large enough so that the entire root system will remain in the dirt you add, then amendments can be beneficial. If you are raising the area less then a foot, then I would recommend that you mix your added soil with the natural soil in that area. The trees will likely do best if they don't have to grow their roots through a soil layer boundary. Drainage can also be greatly affected by soil layer boundaries. If your beds will be large and deep (around 18" to 24") enough so that the root systems will be mostly contained in the added soil, then drainage is all that you will have to worry about concerning the soil layer boundary. In this case, as long as you can achieve good drainage, a very high grade of soil with lots of compost would be best. Whatever you decide to add, be sure it's beneficial to your plants. An example of something that could hurt, is a sand that is not pH neutral. Some sand is not pure silica and can do bad things to your soil's pH levels.

There's a lot of variation between different Japanese Maples. Some will do poorly in full sun, while others will do fine if given enough moisture. Almost all of them will do poorly if planted in full sun and dry soil. Also, some wind protection is beneficial if your bed is in an open area.