Yes, USD and credit cards are accepted everywhere in Costa Rica. There is no need to exchange currency.
Free-Wifi is offered at the vast majority of bars, restaurants, hotels, etc. US cell carriers will also offer daily international plans upon landing in Costa Rica for around $10/day if desired.
Tamarindo is a 1-hour drive from LIR Airport. Rental cars can be booked if desired but we recommend coordinating a shuttle ride to avoid the cost as rental cars are not necessary once you arrive in Tamarindo as the town is very walkable. Golf Carts can also be rented if desired.