Mostel
4360
With Patek there's a few roads to take
The one road is a nice Calatrava ... Your budget would cover this in vintage or almost new...
The second road is chronographs ... Your budget will not cover this.
The third road is complications... Also very much out of your budget.
Since you said 'investment grade' you have to be warned that watches are terrible investments, except in very rare instances, and even Pateks can be heartbreaking to resell if you are learning the hard way.
With that said, based on the minimal info we have, start digging around for a really nice vintage Calatrava.
They will be smaller watches but true representatives of the best Pateks.
Or you can do what most Patek collectors do... Increase your budget exponentially.
Good luck.