Does "the attached updated User Guide" sound wrong?

Both versions are grammatical.

Please take a look at the attached updated User Guide and let me know...

There is no rule which prohibits the use of two adjectives in a row before the noun they modify, as is the case with updated and attached here.

Please find the updated User Guide attached and let me know...

Here, attached is an elliptic form of a subordinate relative clause. The full form is:

Please find the updated User Guide which is attached and let me know...

Which one you will eventually choose depends on your stylistic preferences.

The string of adjectives is fine, but I think you should either put a comma after Guide or start a new sentence after Guide. (Google: comma before conjunction in compound sentences).

Please take a look at the attached updated User Guide, and let me know blah blah.

or

Please take a look at the attached updated User Guide. Let me know blah blah.

You could also reword it:

Please take a look at the updated User Guide, which is attached, and let me know blah blah.

Or

I have attached the updated User Guide. Please let me know blah blah.

Both work just fine because both have clauses with a subject and a verb, making them complete sentences, but as a native speaker of English, the second version sounds better to my ear.

Corrections to both need to be made, though.

1. Please take a look at the attached, updated user guide, and let me know blah blah.

2. Please find the updated user guide attached, and let me know blah blah.

I'm not sure what the details of your user guides are, but I'm guessing that those details (attached, updated) are not important, so I recommend putting a comma between them. You need a comma after "attached" and "guide" because it follows a clause starting with "and." You shouldn't capitalize "user guide" because it's not a proper noun (I'm only assuming this).

Multiple adjectives in a row before a single noun is a notoriously tricky situation for non-native English speakers to master. It's perfectly grammatically correct and does not require any commas to separate them, but certain orders of adjectives sound very "wrong," while others sound fine. Unfortunately, it's difficult to come up with a comprehensive set of rules for the "correct"-sounding ordering, because it's based almost entirely on instinct and experience. See here for a discussion.

But in your specific case, the phrase "attached updated User Guide" sounds perfectly fine to me.