First, welcome to the Guild!
While I've not worked on a Decker grand from that era before, I have seen my share of 1800s pianos (the oldest in my database is currently 1865, not including the squares that I can't find the dates for). I'll basically echo what James wrote. There's a good chance you'll get in way over your head once work starts simply because of the age of the piano. For a complete rebuild (which it likely needs), your client is probably looking in the neighborhood of $25,000-$30,000 or so, depending on a lot of factors, of course. If that number doesn't throw him, get with a reputable rebuilder to give him a more accurate number.
Of course, if all he wants is a semi-functional but mostly furniture PSO you might be able to do some work to it get back to mildly functional. But don't be afraid to tell him that it's beyond reasonable salvage if that's the case.
As far as tuning, you should be able to bring it up to 440 but as James mentioned, be aware of rust, already broken strings, etc. And expect to have a few break if the strings appear to be original and the piano is well below pitch. That's just part of working on antique pianos. The good news is, string replacement is a lot easier to learn on a concert grand than a spinet, LOL.
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Benjamin Sanchez, RPT
Piano Technician / Artisan
(256) 947-9999
www.professional-piano-services.com------------------------------