A complete offer? What a novel idea. All joking aside it's really important to make sure your offer package is complete. If it's not, chances are you will be getting a counter or maybe even no response, which means the property is very much available while your offer is getting cleaned up.
A full offer package includes: 1. The Residential Purchase Agreement and a few disclosures 2. Proof of Funds and 3. A pre-approval letter (if you are financed).
Firstly, having an experienced agent write up your offer can be worth it's weight in gold. If it's not written correctly, it can delay acceptance with a counter or make you miss out on the property. It's also important to understand what terms the seller is looking for, including what is included, excluded, and who they prefer for escrow, etc. The buyer can then take this information into account when putting in their offer, so there is a better chance of acceptance.
Proof of funds are a very necessary component to include up front with your offer, since it shows that you have the cash necessary for your down-payment or if it's an all cash offer, to purchase the property. This can be in the form of a screen shot, bank statement, retirement proceeds, etc. Make sure your account numbers are taken off to protect your security. Any buyer would have a very hard time getting acceptance without showing proof that they have the funds available.
Lastly, if you are financed, it is always a good idea to include your pre-approval letter with your offer. If not, it is hard to verify if you are able to purchase the property. The goal is to make it as easy as possible for the seller to say yes to your offer and accept. If you don't have all the necessary components, you are putting yourself at a disadvantage.
Bonus!! Many times if the seller has lived in the property at some point, or if it has sentimental value (which is most of the time!), it is a nice touch to include a letter about yourselves as the buyers, and explain why you like the property. It can make a big difference to a seller knowing that you have spent some time at the property, noticed the updates/challenges/unique features, and have taken the time to put that in writing. Sometimes it can be comforting for a seller to know a little bit about the potential new owners.