My husband and I just returned yesterday from Nosara. We rented the guest house of someone who was a great host. I did see Harmony while we were there and it is very close to the "tourist area" of Nosara. You will find restaurants, a good bakery, shops, etc. within walking distance but you should plan on having a 4X4 rental. We flew into Liberia and rented from National as they have an office in Nosara in case of any issues.
That said, you must be joking about the roads. They suck, but we drove around on them and enjoyed the adventure. Don't expect something you aren't going to find. The Daihatsu Bego we rented got us around but not without some concern. Don't drive at night especially if you have had a few cocktails. The sun comes up early and sets early too so plan your activities during daylight hours. Get out of bed with the sun and enjoy the birds and the monkeys. Buy a GPS and buy the maps for Costa Rica, without them the GPS won't do it. Many of the restaurants/attractions will not be on the GPS but it will get you to the beaches and some of the businesses. Also, make sure the first thing you get is the "Nosara Map" sponsored by Chiqui's. We were given that by our host and it was VERY HELPFUL. From the airport, use the inland route through Nicoya and farm country. You will have plenty of time to enjoy 4 wheeling the local roads once you arrive.
As for signage ... well there isn't much of that on those dirt roads. We used GPS coordinates to find the house we stayed in, provided by the owner. He had sent us photos of the sights along the road as there is only sign, blue stripes painted on a telephone pole at the turnoff.
Nosara is kind of a remote place. The roads in the condition they are, have kept it that way. If you are going in July, plan to see the turtles at Ostional The road to Ostional is bumpy and the rains can drain across the roadway causing ruts. Go slowly across the ruts. Sometimes you will have to drive just in first gear, but you will get there. That is an amazing place and turtle time is something you should not miss. Also, I recommend River Safari Tours at Boca Nosara. LeRoy and his wife, Kirsten, are very informative and we really liked it. They provide binoculars. You will need to make a reservation and they will be there when you arrive.
Oh yes, you should buy international time on your cell phone if your plan does not include it. This time of year, reservations by phone can ensure a happy experience. Places we liked were Il Pepperoni Restaurant, surfing at Playa Guiones, Romantica Restaurant, and the best boat captain for offshore or inshore fishing is, hands down, Captain Juan Mora owner of the Cowboy. Don't rely on Fishing Nosara for the best. They have the advertising but not the best captain. Many locals will tell you about Juan Mora. If you want to snorkel, go to Playa Rosada and Islita Rosada.
Weather: the green season is much like Florida in the summer ... without hurricanes. Humid and sunny with rain showers nearly every day, sometimes substantial. You will leave with a tan. Slather on that sunscreen first thing in the a.m. and then use the spray-on periodically for the rest of the day. Take a hat, bug repellent just for times when you are outside and they are bugging you, and a cheapo poncho and umbrella may come in handy. Also, keep these items in your knapsack, a dry bag for your phone is a good idea too. Plan to stow your GPS and charger in your knapsack each time you use your car. If you don't leave things, even sunglasses. in your car when you go places, especially the beach, you won't run into a problem. The people who encounter theft usually do so because they are careless.
WE HAD A GREAT TIME! You will too!