This year, I planted in large black plastic containers with a 50%/50% mix of MG potting soil and compost. They thrived in that. I added organic matter periodically (coffee grinds, coco bean mulch, etc.) and fertilized once a month. I also interplanted with Lysmachia ('Creeping Jenny' & 'Jungle Green') to cover the black pots and act as a natural mulch. Seemed to help retain the water quite a bit.

If you do some research on the Internet, you'll find many lists of trees and shrubs for coastal areas, in addition to many good articles on the subject. Here is a link to "Trees and Shrubs that Tolerate Saline Soils and Salt Spray Drift" from the Virginia Cooperative Extension. There is also an excellent publication from US Fish and Wildlife that you may want to search for. Gardening in a coastal environment is challenging but that's part of the fun! Pam Here is a link that might be useful: Trees & Shrubs that Tolerate Saline Soils, Salt Spray Drift from Va Tech

In my opinion, save your money. Skip the bagged potting soil (it's good for containers, but not much use in actual dirt), the bagged topsoil (there's no telling what's in it -- probably what you have already anyway), the peat moss (it doesn't add useful organic matter and is environmentally controversial), and the gypsum (it's good for sodic clay, but we don't have sodic clay). Instead, concentrate on adding organic matter. LeafGro is good. Dehydrated manure is also good -- if it smells, leave the bag open for a few days, and the smell will go away. Put the old mulch and leaves back on. And add more mulch! Preferably something that breaks down easily, like leaves and grass clippings, but bagged shredded hardwood is better than nothing. As Brent says, if you use mulch, you have to do a whole lot less weeding and watering, plus it improves the soil as it breaks down. I do understand your point about flower beds vs. mulch beds, but the problem isn't the mulch, it's the fact that some people apparently would rather look at a nice tidy bed of mulch than an untidy, colorful bed of flowers :-).

First, conifers like pines and cedars dropping needles does not make soil acidic. That is due primarily to the underlying mineral component of the soil and the amount of rainfall the area receives. Heavy rainfall areas tend towards acidic soil. Since the vast majority of plants prefer acid soil anyway, you are probably good to go. With lots of the trees in the area, your primary concern is going to be root competition and lack of soil moisture and a sparsity of nutrients as a result. Big trees have big, aggressive root systems that hog all the goodies. Plants you select to go in this area should be drought tolerant and not have a lot of other demands regarding nutrient issues. Even then you will need to baby them for a couple of seasons with water and fertilizer until they become established. I'll second the Beth Chatto rcommendation. And I'd add The Complete Shade Garden, by George Schenk. Both should have numerous suggestion of plants recommended for dry shade.