By terrace, do you mean a balcony?
Or a sloping hillside that has been terraced?
Or a regular flat-ish garden area?
If you give us some ideas as to the lay-out, size, and sun we can give you some ideas.
Gardeners are very expensive here, one of the reasons I do all my own gardening. But more important, I love doing it. Gardening is a wonderful hobby; good for the soul as well as a fantastic physical work-out.
If the garden is currently very overgrown and neglected you might consider getting someone in to do a clearing, to do the heavy work, which is typically apprentice work - and then do the planting and planning (the fun stuff!) yourself. This would be a lower-cost alternative.
You can hire an apprentice gardener ('ungelernt') but be aware that some firms will charge you for the apprentice's supervision by a qualified gardener - so you pay twice! Speak to the firm you are considering, be very clear about what you want and what you are willing to pay for.
There are indeed lots of independent folks offering gardening help - just be careful of Schwarzarbeit.
The real expense in using a gardener is maintenance work. Few of the gardeners I've spoken with are willing to do any hand work - it's power weed wackers only. Which is why all you see planted here are straight rows of spirea.

To really work the soil, to weed around your more delicate flowers one needs to do the work by hand. At the standard gardener's rate of CHF 75-100 per hour...
A bit more info, and hopefully we can be of more help.