First of all, you can expect to pay anywhere from $100 for a landscape design and drawing on up. It really does depend on the designer, their skill, reputation, education etc. So it is difficult to give you a monetary amount. Basically, pay what you can.
2nd, a good way to find someone is go on the local, or botanical garden tours. You can check out many styles of sun/shade gardening and get an idea of who's work you liked. AND you can often speak to the owner and gardener of the home.
Yes, the designer you choose should be willing to encorporate existing plant material. Although they are being the creative mind for you, keep in mind that you are in control of the cash so you are the boss. They should give you a reason why they can't re-use the materials if they don't use them (eg. for a sunnier yard).
The design can be done at any time of the year.
Personally, I would recommend not going to a nursery and just choosing a designer they select. Lately I have heard many complaints about designers that are "chosen" this way. Really, they are chosen for you, and not you choosing them. Now, if the nursery has more than one design affiliate (so you can interview more than one and it be your choice), or if their house designer has an excellent reputation then I think it's a safe bet.
I would make sure to ask the designer about their background. Ask to see photographs or drive by homes that they have designed, and ask if they have EVER TURNED DOWN A JOB and ask why.
Lastly, check out the http://www.gnps.org/ Georgia Native Plant Society Web page. You can contact someone through their pages. The reason I suggest them is a) there are several good woodland and shade landscape designers affiliated with this group, and b) many native plants of our region are found in woodland areas so whomever is affiliated is a woodland garden designer.

GGG

I've noticed that the Georgia Perennial Plant Association has some good landscape architects in its membership. It might be a good place to check.

Here is a link that might be useful: Georgia Perennial Plant Association

Not sure where you live, but the Spruill Arts Center has a landscaping class, costs around $130 for eight weeks. The guy is really good. You take pictures of your yard and get the size etc and then tell him what you like/don't like, what you want to achieve etc and he will help you design it. If after that you want to go further/more detail he will work with you and provide a full plan for a further fee. This plan will actually tell you how many of each plant you need, where to plant them etc etc. Fun class to do anyway. Someone else on this site suggested it and I found it very useful. Prior to this I tried to use a landscape design person from one of the nurseries and they told me it would be around $250. The guy turned up, looked around and said his fee would be $750. So just becareful how you choose your designer

Quite a few years ago we had a design done for our shady yard by a designer from that large chain of garden centers around here. It cost about $200 but we knew we would save a lot more than that by getting the 2 year 10% discount it qualified us for. The design was quite thorough. It included nothing but very obvious, routine plants, gumpo azaleas, nandinas and so on. It took me a while to realise that there was also one other tiny little problem : her plan depended on the sum rising in the SOUTH. I'm pretty sure it rises in the east ;). I did call and ask her about this, she said it wouldn't matter. Needless to say, the yard now has none of her ideas incorporated in to it. The 10% discount was wonderful, we saved a fortune.

I had similar problems in NE Georgia. My landscape architect had a great eye for design but didn't know plants. Sun plants in shade, shade plants in sun... we asked for deer tolerant plants and it was loaded with deer candy. Seems the key "missing link" is not knowing plants. Maybe if you know someone who is really good with plants, they can help. We paid for the design, but then I researched and substituted plants that were more appropriate horticulturally. We've put in everything ourselves.

Thanks for the input. I am finding some of the same lack of plant knowledge in some of the designers I've talked to. Maybe they want to stick to the safe choices. Has anybody used the guy from the Spruill Arts Center?

I took the class and I really felt it was money well spent. The format of the class goes like this: Everyone goes home and takes pictures of their lot and comes up with a list of their top 5 priorities and a rough budget figure. Each of the following classes is dedicated to drawing up 2 people's plans. It is ad-hoc, as he draws it on the board and you copy it down for yourself, but he labels each plant selection and includes hardscaping and water features if you ask for them. I learned a lot about plant selection from watching other's people's plans, so I got ideas from that too. I took the class about 4 years ago, so I don't know if they still do this, but at the end of the 8 classes, you took a field trip to a very nice nursery at the end of the class. You get a 20% discount at that nursery for a year (I can't tell you the nursery because its a condition of taking the class -- but I believe it is one of the best in Atlanta). Anyway, you can renew the 20% discount every year for $10 by contacting Jim. I renewed 3 years over and I probably saved 3 times the cost of the class.

Jim is a licensed LA, so if you'd like hime to draw up a formal plan, he can do that privately for a fee. I think the formal plan is a true scale representation and it might cover cost breakdowns.

I got Jim's weekly bulletin today and it says his class is starting up in 2 weeks. Here's the contact info.

Jim Collins Landscape class @ Spruill Arts Center 770-394-3447