I had this exact problem earlier this year - I hated my (relatively small) garden and wanted to give it a drastic overhaul but didn't have a huge budget (wanted to spend approx 6k in total).
Initially I decided to try and do a design myself but it was impossible - I could get so far with a 2-dimensional layout, but my garden slopes and I wanted it levelled and it was impossible to work out what the shift in measurements would be. Also couldn't figure out how many steps to draw in, nor how wide they should be etc. I also had an idea of what sort of style I wanted to achieve, but no idea really what would need to be planted to achieve that!
In the end, I hired a landscape designer for 4 hours, and she visited with her laptop and measuring equipt. I gave her a collage of garden photos showing what I liked (raised beds, sleepers, architectural planting, relaxation areas, subtropical rather than English country garden etc). I also gave her a list of things I didn't like about current garden, plus another list of things I wanted the garden to have, or be able to do. From this, she drew up a proper design, and pointed out things which were going to be too expensive to achieve (eg curved landscaping much costlier than geometric lines)
She also did a planting plan, then I got both the garden design and planting plan printed onto A3 and laminated, then got quotes from contractors to do the work. The design evolved a little bit after that, but not drastically, and by early summer I had the garden of my dreams.
The exact plans were really important for the costing up as the bulk of the costs came from shifting earth (I had a digger in my back garden for 2 days!) and the cost of the timber for the raised beds.
It sounds like a similar approach would work well for you, but I guess it's a case of finding that person working in your neck of the woods!