This is an area frequently misunderstood. There is no magic division between "documents that are contracts" and "documents that aren't contracts" - contracts are contracts by nature of the information recorded, not their format, or whether they're called contracts. Bus tickets are contractual documents, for example.
Some contracts, such as those for real property, have formal requirements in order to be deemed an enforceable contract. In the case of land, the legislation requires that contracts for real property be *evidenced in writing*, but that doesn't limit it to standard form contracts. Provided certain information is recorded in writing - such as a description of the land, the price, the buyer and seller, and a few other basic terms - and both parties have signed, it's a binding contract for transfer of land.
So if you make a written offer, it is likely to contain adequate information to create a binding contract if counter-signed by the vendor. Even if it's a letter and the vendor just signs and dates underneath your signature. It's the information, remember, that's important - not the form.
Submitting an unsigned contract with a signed cheque attached is also likely to create a binding contract under the joinder doctrine, whereby documents can be "joined" together to fulfil the formalities requirements. (So the fact that the cheque and the unsigned document are clearly "joined" may mean that your signature on the cheque is as good as a signature on the document, from a legal perspective.)
I think one clear way to avoid finding yourself bound against your will would be to include a statement in your letter of offer that the parties do not intend to be bound until the preparation of a formal agreement, as in
Masters v Cameron). I am not a lawyer and would not presume to word such a clause myself, but I would ask for legal advice on an appropriate clause to include in your offer.
No, an accepted offer *is* a contract. (As always in the law, with some exceptions.)
Yes, unless you protected yourself with some wording as I alluded to above, that you've had approved by a lawyer. Otherwise, offers are considered available for acceptance until revoked, or lapse of "a reasonable period" - and who wants to fight out in Court what that is?