If you haven't already, be sure to take a look at this thread on the COP issue. It's probably the most common problem that seems to hit every retro bird eventually.
forums.fordthunderbirdforum.com
If you have the opportunity before buying, see if you can get a look under the plug and coil covers--particularly on the driver's side to see if there is any water intrusion into the plug wells. Had I known to do this when I bought my car I may have asked for a lower price. Fortunately, this forum has tons of info on how to address the issue plus links to suggested replacement coils and plugs.
Hope you make a good decision!

Top Issue- 02-05 Ford Thunderbird Ignition Coils COP
2002-2005 Ford Thunderbird and Lincoln LS have a widely known issue with Ignition coils going bad and also fouling out spark plugs causing misfires . Just by replacing these components, you may be able to bring new life to your engine! At the time of this post, 59.30 for an entire set of...

If you have the opportunity before buying, see if you can get a look under the plug and coil covers--particularly on the driver's side to see if there is any water intrusion into the plug wells. Had I known to do this when I bought my car I may have asked for a lower price. Fortunately, this forum has tons of info on how to address the issue plus links to suggested replacement coils and plugs.
Hope you make a good decision!
True that. It's rare to find a new electronics module for any of these cars, and lots of problems reported point back to the modules. There are at least three main modules in these cars. Most people end up getting them repaired since new or NOS are hard to find.Guess it depends on your definition of serious. I’ve known of several cars that have gone 200K with no blown engine or tranny. However any of the various electronic modules can go at any time with little or no warning. Repairs to those can get pricey.