Help! Which 90's Buick LeSabre or Century should I buy ?

I would like to buy an Early, Mid or late 90's LeSabre or Century
I have about $1500 to $4000 to spend.
I need a crash course in what to look for !

Any known problems to watch out for ?

Is there a certain mileage that things start going wrong ?

Should I be scaried away from an 'over the average' miles car ?

Is the Intake manifold problems on the LeSabre 3800 motor manageable for a driveway mechanic ?
Where on the engine can I look for the beginnings of trouble ?

Should I just stick with the 3.1

Is there a certain letter in the VIN I should look for to ID a bad engine.

I'm leaning toward the LeSabre because of body style but
having owned a '87 Century 170k, I know Century are dependable.

So many ????? But Information is Knowledge....

Any help appreciated
Thank You