the 2004's had a lot of problems. I would try to stay away from 04. If it is certified at least you know it will be covered under warranty, but you'll have to be OK with the trips back and forth to the dealership.
Automobile(s): '08 STS-V Sedan Black Formerly three CTS-V's, XLR and STS4N*
Re: looking to buy o4 XLR
Not to confuse this but I don't recall "a lot of problems" with the 04's. I know that they did have some of the typical new model issues that have been fixed by tweeks through the service process. The drivetrain is stout and should not be an issue for many miles. Buy certified and you are protected. If I remember there is some information of typical 04 issues posted deeper in this forum. Perhaps a search will help.
There are some great deals to be had on '04 XLRs. First, if a dealer is selling it, have the salesman call up the VIN's repair history. This will help you determine how much of a risk you're going to take on the vehicle. My car had the radiator/water pump replaced twice; other than that, nothing major. Ensure the top is operating correctly by having the mechanism cycled however many times it takes to satisfy you. If the car has been sitting outside on a lot, open the trunk and check the carpeting, front to back. Open both trunk side compartments and look for any evidence of water. After an extended period outside, (1 year!) my car was wet, but 18 hours with a heater in the trunk with the carpet rolled back fixed that. There is a TSB out which details how to properly repair/seal leaks in this car. For piece of mind as far as price is concerned, go to Autotrader.com and look (countrywide) for XLRs similar to the one you want. Mine was in the top five (lowest price) out of 500+ 2004 entries, so I felt the price was fair. The only thing mechanically wrong with my car was both seats didn't heat/cool properly. That took about five minutes to figure out --the vent tubes on both seats were disconnected from the seat movement. When you think you find the XLR of your dreams, take the time to make a list of each switch and it's proper operation, using available online Owner's Manuals as a reference. Take your checklist, sit in the car and operate everything to ensure you don't have any surprises later on. I did extensive research before my purchase and other than radiators, couldn't find any major issues with this vehicle (unlike the CTS-V which has a glass rear end.) Once you buy your car, go to gmpartsdirect.com to buy any parts you want to make the car look brand-new. You'll save 40% or more. Do the research and you'll be less likely to get burned, and more likely to congratulate yourself later. I hope this helps.