If you're willing to sort the documents as suggested in the comments by Edward Nunn, then my answer here should be of help to you:

Document Scanner with specific modes

You can definitely find cheaper units that don't use features like Auto Document Feed (ADF) and whatever else you don't need, but if you need to get this project done fast in a business setting then I'd recommend something with ADF at least. Considering the apparent scale of your project, the cost of additional man hours put in due to the absence of those features would probably approach or exceed the cost of just buying the scanner.

#Original Question From Link:

We are in search for a business document scanner with following scan options, all done over a ADF.

Simplex: front side

Duplex: front and back side

1. Simplex scan of every sheet to its own PDF.
2. Simplex scan of all sheets to one PDF.
3. Duplex scan of every sheet to its own PDF (One PDF with front and back side).
4. Duplex scan of all sheets to one PDF.

We talked with a supplier of ours and got a Brother ADS-2800W from him, but its incapable of mode nr 3. After some research and getting back with the Brother Support, none of Brothers Scanner is apparently capable of nr 3.

Of course we also tried the usual ways (google, product comparison websites and all those "Best Scanners of >>insert year here<<"-Reviews) but none of those gave the information needed or didnt comply with at least one criteria.

Criteria:

• ADF
• A4 sheet size
• Modes listed above
• Desktop size
• WLAN / LAN
• Save to SMB Network Share
• Price of 600€ / $ top

Thanks in advance for any kind of help!

#Full answer below:

My office recently grabbed an HP ScanJet 3000 S3. It's a bit above your price range (closer to $800-900), but I've been in charge of developing our scan process and I can tell you with confidence that it does everything you listed.

We specifically got it to digitize old paper records, and it is set up at a small stand-alone desk with a computer.

• ADF = Included
• A4 sheet size = Included, also has other custom paper sizes. Smaller sheets can sometimes have trouble being grabbed by the feed mechanism, but it's functional.
• Modes listed above = All included.
• Desktop size = Fits on a pretty tiny desk with plenty of room to work. Very space efficient.
• WLAN / LAN = This feature exists but you have to buy a $40 wireless adapter that is made special just for this unit, and you can only scan to mobile devices using this method. I know don't even get me started right now, but this thing works and finding a wireless workaround solution was extremely simple since we work from a server platform. We returned the stupid $40 USB dongle.
• Save to SMB Network Share = Included.
• Price of 600€ / $ top = Nope.

It's far from a perfect solution and you usually won't find me recommending products that don't fit the user's question, but we had a particularly long and arduous experience trying to find something that would fit our needs. I see similar conditions in your post. We had to make a few compromises, but at the end of the day we needed something and this little headache of a scanner made our project possible. Perhaps it can do the same for you.

I can definitely tell you that unless you're looking for something used, duplex auto-document feed scanners that can handle even a 50 page count stack at a time don't exist for under $600. Well, they may exist, but there wasn't anything I found that was suitable and trustworthy enough for business use.