There are a ton of F3-HPs out there. On the 'Bay the winning prices for same are all over the place; if you buy one and can sleep at night knowing what you've paid, that is all that matters. Someone will certainly have paid less than what you did and someone will certainly have paid more. Take a lot of pictures and move on. Nothing is a "good investment" except in hindsight, but on Ebay F3s with the DE2 usually fetch less than F3s with the DE3. You might consider that a hint from the Market.
Having said that, if the box you're looking at has the motor drive connector cover and some brassing on the bottom I would say it's likely that it rarely if ever had a motor on it. That's good news for the shutter. If you have large hands you might like it with an MD4 motor; there are lots on that auction site, cheap. (They might even have an extra cover in the little storage pocket. You'd be surprised how often people forget that's there.) If you have large hands you might like the feel of an F4 better. I don't so I cannot speak to that. I have an MD4 but never use it except in winter when it makes the camera easier to hold with gloves (and the drive's batteries run the camera, rather than the camera's).
Finally, Nikons are very Hemingway. A mint Nikon is pretty. A brassed up Nikon is lovely.