Ex garden designer here.
You’ve made a good start by maing a list of what you want from the garden – that’s the easy bit. Have you done a survey of the garden yet? You can’t really draw up a design unless you know how what shape it is & which way it points. A basic technique once you’ve done the survey is then to do a simple ‘audit’ of your garden – make a note of any good/bad views, where the sun is in the morning/afternoon, and dry/boggy bits etc.
You can then go on to start putting a plan together. The problem most people run into is that they start by thinking about features/plants rather than making the space (argh – designer word!) work. If you think about designing a house then you’d start thinking about the arrangement of rooms & how they link together,rather than getting a blank piece of paper and saying ‘i want a sofa here and the dining room table over there’. So that’s what you need to do with the garden. Work with the functional space (argh – 2 designer words!), then planting will fill the stuff that’s left over. If you check John Brookes’ books you’ll find his ‘grid system’ which is a decent way to ensure things stay related to each other.
Straight lines tend to boring so avoid if at all possible.
Nope, straight lines have their place in the garden – Titchmarsh style wobbly hosepipe borders can look a right mess, and (IMHO) the smaller the garden the less space there is for curves.
Level changes can really work too – even going up or down a single step can make the garden feel like two different areas.
Loads more, but as a beginner I’d suggest having a nose through some of Brookes’ stuff and see where that takes you.
As for getting a landscaper in… well, back to the house analogy. There are some jobs where you know what you want and you’d just call a builder, there are some where you need a bit of input and you’d get a design/build type person involved who would at least do some drawings, and for a full house build you’d probably go to an architect (or building designer). Same deal with gardens.
Above all, have fun with it.