Before you blame the transmissions for those bucking problems in that range (35-45mph) do a complete filter and flush and buy the mercon oil at Ford. It is the recommended fluid and a flush sometimes solves these problems. Then change every 30,000kms. Its not guaranteed to fix the problem, but these trannys may exhibit problems with dirty oil. A quick check will tell you. Red is good. Deep red or brown with a burnt smell means new oil and filter. I have a 1994 CVPI and it works really good. Not real fast, but I have highway gearing in the rear end. Nice on turns and when loaded up due to the heavy duty gear underneath. Will definitely last longer than a LX model if you get a Supervisor or detective car rather than regular patrol.
I also have a 1994 CVPI which I bought used in April 2007 with 175000 KMS. My car was obviously maintained well by the police department as I am still running it and have only put on a battery post ($2.50), 2 front tires, and a few oil/filter changes. I run this car like its stolen most of the time and still after over 35000kms it runs great. Also not super fast as I have 3.08 rear end but not too bad on fuel and awesome on turns but rough on bumps due to heavy duty suspension. Get one of these if you like to run a car hard. They will definitely be able to take it.
I own an '03 CVPI, ret'd Border Patrol car. When I started looking for a Cr Vic, I wasn't even thinking about a "Civilian" base, commercial heavy duty, or even LX Sport Vic, I wanted a Police Interceptor. Everything is heavy duty, extra heavy duty, or severe duty. Police shock absorbers, aluminum police driveshaft, etc.
As for the check engine light, instead of having the dealer fix it, I would go and buy a scan tool to see what the problem was. Could have just been a misfire or water in the fuel. At which point I would fix it myself, avoiding those ridiculous service fees, unless it couldn't be fixed or cost too much to fix.
As for the trans, it's probably like the other guy said, fluid and filter. Although being a PI, that may be normal, shifting into 3rd gear. It's a heavy duty vehicle, don't expect a dead quiet from it. Mine kinda whirrs from 5 mph till it shifts into 2nd, again that's normal.
As for the seat, It ain't that hard to get some seats out of a Vic, or a Marquis, and put them in. Sounds like you might need to learn to work on cars, or at least check your fluids at every fuel-up, or once a week. For oil, check when engine is cool. for trans fluid, check while engine is at normal temp, idling in Park. You know, if you take care of any car, it should last 40 years or more.
Yeah that whine in first gear is normal. I have a Grand Marquis and Crown Vic, and they both do it. Man I love that sound! It's like a jet.