As far as I know it is just a regular ol' mustang with a 6 cylinder engine. Might be all original.

What are some reliable sites or ways for getting a value? Online values range anywhere from 6k - 25k.

What is the best way to handle exchanging that much money besides cash?

Thanks for any tips.

Check the website Bring a Trailer,  id avoid Mecum and Barrett Jackson auction prices,  they ruined the classic auto market. Can try Classic Car Auto trader

Might wanna research that further,  my 10 seconds on WWW says it could be rare

https://stevemckelvie.wordpress.com/2018/12/30/the-1969-mustang-e-a-very-rare-mustang/

I killed a '69 mustang once.  :(

My moms car, only a couple years old.  Slid on a muddy road. Hit a tree, rolled it down a hill.  Totaled.  302v8 automatic.

Damn kids.

I never heard of the Mustang E before.  Only 50 of them made? OP you might be sitting on a gold mine.

If I had to guess OP had a 69 mustang "Grande".

Grande were the usual coupes and non-sportroofs.

Go over to:  Vintage Mustang Forum
Someone there will be able to tell you exactly what you've got, or point you to someone who will.
Bring photos to help them out with the valuation.
Go to the Vintage Mustang sub forum.

As far as handling money, just meet the buyer at the bank and have him get a cashier's check for the purchase price.

Need more information to make an opinion. What transmission? Fastback? Coupe ? Options ? Color ? Rust ? Runs and drives?

This . Could be a rusted up , non matching numbers car , with a swapped in automatic when it came with a manual  . Everybody thinks they are sitting on a priceless one off car , when in reality it's not what they think it is based off their " internet research " .
. What's wrong with getting handed up to 25k cash ?

I have a friend who's step Dad had a super rare Shelby. It had been mostly stripped down to the frame and left out in a pasture for 20+ years. Well someone told Mustang magazine about it, and they came out to take photos, and question the step dad about it. Right after the article came out, some collector called and asked how much he wanted for it. Typical Texan, he said "Everything I own is for sale, but can you afford my asking price is the question" and hung up. (Guy was pushy and arrogant on the phone) The guy shows up a few days later unannounced and offered $80K cash. Step-Dad said " I might have considered it if you hadn't waved your cash in my face without telling me you were coming. It's now the one thing I own I will never sell to you".
After step-dad passed away, my friend sold it for $100K cash to a different collector.