If the operable term here is "small maintenance budget," an '88 735 is not for you. The previous reply is correct -- a lot of electrical gremlins in '88s and '89s that improved later. If the dealer is not a BMW dealer, I would doubt his ability to activate the computer. Radio does need code as mentioned. Only headliner fix is to have it replaced.
Vibration at idle could be drive shaft ($1000 job). Other things you might have problems with considering age and mileage: entire front suspension, a/c system, transmission, exhaust (check replacement prices).
BMW 7s have fabulous rides and many features not on American cars 10 years later. But you ought to budget repair and meintenance at several thousand dollars a year until everything is right on it.
My experience base: '85 BMW 535i (few electrical problems, great car); '89 735iL (many electrical glitches, several $thousand repair in four years); '96 750iL (virtually free of inherent problems, but it ought to be for what it costs).
It's hard to tell at a distance, but $6,000 for an '88 is more than I'd be willing to consider. But once you've driven a BMW 7 for any length of time, you'll never be satisfied with a Cadillac. Sorry this post is so late --I just joined.
Jim