So, as of this writing, the 91 Park Av is going on 18 years old (since it was a 1990 model year).
I am hard pressed to think of any sedans from the 1990/1991 model years that have an aesthetic which remains fresh and sharp today - other than this one. Even almost two decades later, they still look great and contemporary, but most importantly, there's a reason you still SEE SO MANY of the 1991-1996 Buicks on the road today. They're just fantastic automobiles!
The GM 3800 3.8 was rated in Wards Top 10 Best Engines of the 20th Century for good reason. If you take care of them, they're absolutely bullet proof. The 3800 Series II as found on the later models is even a bit better, but let there be no doubt - the motor that drives your 1st Generation Park Av is as reliable as anything coming out of Germany or Japan. A well maintained 3800 can reliably be expected to get 200K or more, with plenty of examples going well into the 300 and even 400,000 range.
Gearhead stuff aside, these are about as smooth and comfortable a ride as you're going to find.
Anyone reading this today is someone who is looking for a lower priced, older model used car, which the 1991 Park Avenue now is. If you find one that has been carefully maintained with relatively low miles, you absolutely aren't going to find another car in this price range with this level of comfort, reliability and quality. For the couple grand a very well maintained 1991 Park Avenue brings, you're often getting a car that could have another decade left in it.