First off, these are really great cars! I have a '93 740iL and I absolutely love it! At 110,000 miles is little more than broken-in with many, many great driving miles ahead. However, they are somewhat expensive cars to maintain.
You might want to look at some of the things at this link:
http://www.e38.org/ One link is to a buying guide; another is to a list of things to be done at Inspection II.
On my '93 740iL there's an upper and lower oil pan. On one of them (upper?) the gasket is easy to change but very difficult on the other. If the quantity of oil leaked is not significant I wouldn't let that stop me from buying it. Sorta depends on your tolerance for a stain on the driveway.
Another source of leaks on these cars is the valve cover gaskets. Ask the mechanic about these and maybe get a price. I have a write-up about changing them if you have a pretty good tool set and the confidence. I'd guess the BMW dealer would charge $500 to change these.
I'd guess that Inpection II from the BMW dealership would cost $1500-$2000. If you check, please report how close I am. The big thing is that the 740iL cars use a "liquid gold" transmission fluid which costs around $100 gallon (from the dealership) and you'll need several gallons. You can buy the fluid for maybe $60 per gallon via other sources and have your mechanic change it. (At 110,000 miles it SHOULD be changed even though BMW originally claimed the fluid was a lifetime fluid. I emailed the transmission supplier, ZF transmissions in Germany, about this and they reluctantly admitted the fluid should be changed at 100,000 miles.)
Again, I really love my car and think it's worth the extra maintenance costs vs. an average car but I wanted to let you know these things. Given that the car needs money spent on it right away you might want to negotiate a bit on the price.
Good luck and please report your findings so we can all learn.