You've gotten really good explainations. I think if you are relatively
inexperianced you may be frustrated though impulse buying (and
inapropriate situations). That advice "buy plants suited to clay" is
best because each maginal plant will perform differently and you will
have no idea what you've just done to improve the soil, as you never
planted it yet.

simple is better, yet mass planting of one type of plant could be a
huge boon or a miserable loss. I'd try to avoid shade plants in shade
at first, unless you know where the water lies, or buy those plants
that do well in soil that never ever dries out, eg bog plants, and
make a bog. Astilbes should be safe in a fresh loamy mix, they are
nice too.

(I like to talk, i garden in sand here though, good luck)