Michelle, for scars I've found vitamin E works well when applied topically. There's plenty of over the counter topical applications out there for that. Stri-Vectin also works well for scars, stretch marks, discoloration etc, you can get it through a dermatologist or even at dept stores like Dillards, it is pricey though. Bio-oil also helps as does rosewater when mixed with other things (?), my mom uses that on her decolletage for sunspots and red spots associated with aging and it seems to help her.
As for what to use for lotion, cream or oil, try to purchase a quality massage cream, there are plenty of great suppliers out there just gotta look through them to find what's in your price range. The advantage of massage oriented products verses general lotions or creams is that it doesn't dry up as quickly when on skin and has much less friction and doesn't clump or pull off dead skin as much as normal lotions do.
If you're looking to mix EO's in with a good base oil, look towards a stable base that doesn't go stale very easily such as Jojba oil, it's relatively inexpensive and has a good shelf life. I'm not sure what EO's would be helpful towards increasing growth in the breast tissue, but that's what research and girls who've been at NBE may have a bit more insight on.
Just research, research, research and then try for a good amount of time as you would with any other method. See what works, maybe even try to set a control of just plain oils verses infusing base oils or creams with EO's.
Hope that helps!