This isn't as intense as you think. If you've ever bought a car, you can handle buying a piece of property.
A letter of intent is really nothing besides writing a letter that says, "I intend to buy". It's a nice psychological tool to indicate a level of seriousness. Think of it as sending a lady a note after a date rather than talking to her the next time you happen to see her. You're indicating seriousness - that's all.
IMHO. If you're making an offer, make it with a purchase contract - not letter of intent. Actually the best thing is to sit down with them, talk numbers and when you've both agreed verbally, put it in writing and sight. Make it a conversation between yourself in the seller....all the contract is, is a binding confirmation of your agreement. Earnest may or may not be needed depending on what contract you use. Realtors like earnest more than sellers because realtors livelihoods depend on a transaction closing so they want you psychologically locked into closing. (Watch the realtors jump in now and tell you how important earnest is.... ;-) ).
Tim