I did a type 2 inspection on the same car. It ran me about 700 dollars. I can provide the list of everything they did, if you want, sometime tonight.
For me, the gas mileage is 17.4 MPG between city/freeway. I'm 19 and have a heavy foot, so you should get at LEAST this.
The biggest issue of the 530i/540i is the Nikasil problem. Here's a link: http://www.lestac.co.uk/bmw/nikasil.htm
Basically, the cylinder walls in the M60 blocks -- the 4.0 V8 in the 1995 540i -- is lined with Nikasil. It reacts very, very badly with gasoline containing high sulphur, boring through the walls, and killing the block quickly.
If you want to know whether or not a particular 530i/540i faces this problem, the best thing you can do is pay CLOSE attention to the idle. Let the car run for a few minutes in neutral, open the door wide, and hold it, and if it's shaking, there's a problem.
Also, do a compression test. That will tell you whether or not you are facing the infamous Nikasil issue.
If you encounter an E34 540i, and it has 100,000 or more miles, and the compression test comes back favorably, then you shouldn't worry. Just put premium gas in it, and you'll never run into any problems with the Nikasil.
E34 owners on bimmer.info have run Nikasil blocks up to 200,000 miles. Just be cautious about the issue -- not overly paranoid.
You can easily buy an E34 540i with a six speed for fifteen grand. The problem is, FINDING one. They're fairly rare cars, being that they only made the E34 with the 4.0 V8 AND the six speed in one year in the States -- 1995.
I pay 200 bucks a month for full coverage on Geico. No accidents, no tickets, been driving two and a half years. That should give you a rough estimate of your insurance.
Also, since you have 15K to spend, look to see if you can get a 1997 540i with a six speed -- first year of the E39. Personally, I prefer the styling of the E34 over the E39, but most would opt for the newer 5-series.
Good luck with your purchase.