Your Trane quote is a 13 seer. not a 14 seer.
The aluminum coils are an improvement over copper for several reasons. The only downfall to aluminum coils is not many people can braze aluminum.....so they automatically call it a bad idea. But the alum. coils won't corrode out as quick as copper.
As fas as unit size, no way of telling from here. Proper load calc and duct design will be the determining factor there.