Lots of questions...
Make sure you get any and all manuals, spare parts, old parts, documentation, etc. the old owner may have squirreled away. In particular, if a carb rebuild was done recently, see if you can get the old parts -- many rebuild kits are junk, and lots of us have learned the hard way that cleaning and reinstalling the old jets is the way to go.
As far as manuals, there are Clymers, Haynes, and, the rarest bird of all, the original GS850 Suzuki shop manual (I've heard they exist, but I've never actually seen one in 10 years of GS850 ownership). Buy all the above if you can, and always read them with a skeptical eye. Don't trust them one bit. All three are incredibly obfuscatory, incorrect, and outdated at times. By reading different perspectives, you can sometimes get a better idea of what's happening. Get the Haynes and Clymer manuals and a microfiche here:
http://repairmanuals.com
Ask about the age of all the standard stuff you have to replace every 20 years or so, including intake boots, intake boot o-rings, the petcock, etc. Ask about any upgrades to the suspension -- modern fork springs and shocks are mandatory safety upgrades, and if you're lucky, they will already be in place.
Ask about any luggage and/or fairing pieces that may go with the bike. Even if you prefer to ride a naked bike, vintage luggage and fairings are quite valuable.
Inquire as to the general health of the electrical system, any upgrades, etc. Check the stator connectors -- ideally, they've been removed and soldered to the wiring harness. More commonly, they are blackened from the heat generated from a poor connection and must be soldered ASAP.
If the seat is haggard, you can revive it with a new seat cover:
http://motorcycleseatcovers.com
Ask when the last valve check and oil change was done. See if you can verify this. Valve checks are required every 4,000 miles, and they are NOT optional. Many people have a magical belief that they can defer valve checks indefinitely. GS valve clearances tend to decrease, so you can't wait until something starts clattering before checking. Oil changes are called for every 2,000 miles, but many here are comfortable going to 3,000 or so with higher quality modern oils.
See if the airbox covers have been sealed with foam weatherstripping -- this is something every GS850 needs at some point, and if the PO already took care of it, great.
Some states require notaries to preside over title transfers, so be sure to find out and follow PA law. The title should have instructions on it as to whether a notary is needed -- no big deal, just head to a bank or credit union branch or maybe a library and pay the nice lady a couple of bucks. Do NOT leave without a properly signed and notarized title.
Do NOT allow any shortcuts here -- if your home state DMV finds the SLIGHTEST irregularity or, God forbid, an attempted correction, they will refuse to process the title. Then you will have to somehow convince the PO (who already has your money, and is thus entirely unmotivated) to go wait in line to apply for a duplicate title and start all over. This costs money and can take weeks. If the title is damaged or incorrect in any way, keep your money and go back home until the PO takes care of the problem. You do not need a title transfer service.
Lastly, get a bill of sale. Google around, and you can find bill of sale forms you can print off. Bring several copies with you in case one of you makes a mistake, and fill out and sign two copies while you're there. If you make a mistake on one, start over -- don't alter the bill of sale. Sometimes my DMV wants to see a bill of sale, and sometimes they don't. Best to have it ready.
