I don't think there are a lot of issues on that generation Seville, and if they had some, I think they'd have the majority of them figured out by '91...the 5th and last year of production. Like Doug said, keep an eye out for leaks, just like any other 16 year old car, they'll have their share of those. But other than that, the 4.9L is a rock solid V8, but the early ones are more suseptible to the 4.9L main bearing thump...my '92 has it.
Otherwise, I think they have the computer command ride, which'll mean expensive OEM replacement struts, but I'm sure you know about that, your '99 has a more advanced and expensive version of them.
Other possible issues.....digital dash problems perhaps? I know a lot of the digital dashes used in '80s GM cars had a lot of problems but I can't remember hearing a lot of bad things about the one used in these cars.