I agree with Larry. You will spend money hand over fist, making an abused/neglected car nice. I drive a 1989 560SEL, and it is a nice one, but I have spent well over $2000 in parts to bring it to great condition.
any ticking in the rockers? new rockers arms are $20 each, and the hydraulic compensators are up there too. Plus the tool to pull the rocker arms costs money.
timing chain, cam sprockets and rail replacement - $800 -$1200 at shop
front end work? could run $1000 easily at a shop, replacing everything and then align.
Rear suspension - accumulators, any leaks in hydraulic lines, etc. (fluid is $24 a liter!!)
A/C repairs - don't ask! check the high pressure hoses off the compressor for oily residue. The dual hose off the compressor is $350 itself I think! Then add 3 lbs of R12, unless you want to convert it. evaporator repair is a big dollar item, although not common in that car.
brakes - $45 - 50 a rotor, and pads are fairly reasonable. But if you pay someone to do it, expect to spend real money.
heat - monovalve repair kit <$20. installation - $??
exhaust - Is there rust there? I do not consider rust, as most of the cars here are clean. (I am spoiled by never breaking loose a rusted bolt in Florida) the exhaust system is a big item to replace if rusted.
Sunroof repairs - bridge angle kit is cheap - $200 I think? never did it myself. but again a shop will charge you to do it.
Belts, hoses, new fluids, misc things you will have to do right away. I guessed about $200 on mine.
Dumb question - do you own metric tools? add some dollars for that. Then remember to research the special tools for jobs like the timing chain and rocker arms.
$8300 sounds like a deal for a 560SEC. Keep in mind, MBenz cars SELDOM end up in junkyards....
If you can fix things, great, but if not, be cautious! It adds up fast!
Good luck and have fun!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)