Will I be deported due to my overstay or can I go ahead and marry?

If your fiancee is a U.S. citizen, go ahead, marry her, and retain an immigration attorney to prepare and file the necessary papers. If she is a permanent resident, the situation is more complicated, and you should take care to not stay in the U.S. more than 180 days after your non-immigrant status expired. Many people mistake the visa expiration date and the status expiration date. The expiration date written on your visa is the last date when you could have entered the U.S. on that visa. When you entered the U.S., the Immigration inspector put an oval stamp with the words "Admitted".

You can go ahead and get married and file for your permanent residence after you marry. Until you actually file, however, you are in the US unlawfully and subject to removal if you are discovered, so I would suggest that you get married and file as soon as possible.

Generally, in this situation, if you marry a USA citizen, yes, you can stay and file all the proper immigration papers.

Had you entered on an actual non-immigrant visa (i.e. B-2) and overstayed and then married your USC spouse, you would be able to seek adjustment of status and have been granted a waiver for the overstay period. Unfortunately, in your specific circumstances, since you overstayed on a VWP (no actual visa) and did not file for spousal adjustment of status within the 90-day period granted you under VWP (a very small window) you will be required to consular process in order to move forward on any USC-sponsored effort to seek permanent residence based upon your contemplated upcoming marriage. You should plan to depart prior to 180 days from your last authorized date under VWP before you trigger the 3 or 10 year bars under IIRAIRA if you seek to return under your future wife's sponsorship.

You can apply for adjustment of status only if you are married to a US citizen. You must submit your applications before the government tries to deport you.

You will be fine. You can overstay a visa waiver and marry without any problems. The entire green card process takes about 3 to 4 months from start to finish.