My take on this has been to look at what the older FF camera actually produces via flickr etc, and if I like what I see, then why not? It doesn't become any less good than it was just because it's older.
For years I said there was no noticeable difference between a modern APSC (Fuji in my case) and my older FF (Canon 5D & 5DIII), but eventually I convinced myself there was (I still don't know if it's not just my imagination!), and moved back to FF (Sony).
The one thing you will almost certainly be trading off though, and forgive me as I don't know the Nikon system in any detail, is the 'quality of life' advances that would have crept into a later system (more focus points / eye AF / Speed of focus / better ergonomics / more customisable buttons etc).
If you use manual controls and set ISO, shutter and aperture yourself then a more advanced camera than the d5000 range is a big benefit.
Low light shooting a FF usually has a stop or so of better performance so depending what lens you have that advantage may be eroded ( like if you have an F4 lens on a FF and a F1.4 on an apsc )
In the case of a d800e you will notice 36mp alot when you crop or zoom in or make a large print. No difference on a small computer screen of un-zoomed images.
I would get a d7500 instead, they are £650 with e-infinity now.
You will only see a difference on your screen at 100% and I wouldn’t care to guess which would give better results - the biggest difference will be down to the lens.
Some people like very high res best of the best digital files but I’ve long since stopped worrying about that as ANY half decent modern camera will give you fantastic prints up to a size bigger than most will ever print.
Nowadays I reckon the main reason for getting FF is to get a shallow DoF look, which at the extremes you can’t do with smaller sensors.
Out of those I’d go for the D800E for landscape but you’d obviously need to get full frame lenses if you haven’t already. Not sure if the autofocus would be ideal for wildlife though, maybe others would know.
I have owned in the recent past an original 5d aka 5d classic. Currently using eos 1ds mk2 full frame and loving it. The buffer is poor and slow the screen is small and it’s a pain using the menu with the two button presses. It’s built like a brick but it’s a fun camera to use and I am very happy with the results and can be picked up second hand between £300 to £500. The 5d is also great fun and if you have some nice wide angles which are non Efs they will open up a new world if you have only used crop bodies.
What's the shutter count? Seems very cheap and probably well, well used! I sold one 2 years back for twice that, maybe they have come down a hell of a lot but that still seems cheap. Mine had only 22K on the clock. Stunning camera if you go for it anyway, I only sold on because I wanted to jump on the mirrorless wagon.