base your planting on the location of the garden, place sun loving plants in the sunniest parts of the garden and use shade loving plants in northern facing areas...check the labels of the plants for this information.
Azaleas are loveley but check if your soil is suitable.
Also remember to leave enough space between your plants, it might look bare at the start but give it a year and it matures quite quickly especially if you are planting phormiums.
Mix evergreen and decidious plants so that you have even colour through out the garden.
I find add stones (10-15mm) as a border with grasses and silver spears looks quite nice, a coloured stone like yellow sandstone is nice and bright to look at the winter rather than the traditional dull drive pebble stones and also adds contrast against the coloured spiked leaves of the phormium (go with as many varieties as possible to give different colours and shape).
Not a fan of the tree fern, always looks lovely in garden centres but it never does well usually in gardens.
Other shrubs which are nice and durable are dwarf cherry blossoms, fuschias (different perenial types are best), conifers of different shapes and colours for all year round colour.
Our neighbour has banna trees and there leaves get torn very easily and don't look nice so plant in a shaded area is best, nice for colour though but they get big so leave enough space.
If spending alot of money i wouldn't go to the big garden certrers (B&Q, Home base, Woodies etc)as you'll be ripped off, go to these first to get an idea of what they charge and then go to a local garden shop, I have noticed from my travels that plants a very expensive in lenster compared to munster, i visited one shop in kildare and they were charging 21 euros for a tiny phornium, where in cork i can get a plant 3 times the size for 8 euro's, i have heard of a place in tipperary that is supposed to be very good for price and plant variety and will travel, will get the name if interested.
Best of luck, but remember to make sure that you have the ground work done correctly first.