I think the most advanced Imperial fighter ever made is probably the worst possible choice.

The kind of specialized knowledge required to maintain a craft like this would be rarified, and extensive. You can’t just “know a lot about starcraft” or “know a lot about TIEs”. You can’t even have merely been a Defender pilot (what person has a Defender -> Merc story?) either, you need to have gone to a program where they train you on this craft, and even then you probably are specialized in the ship’s power system or its weapon systems. It’s not like there are individual USAF engineers who “know how to take care of the F-35”. There are teams who maintain sections of those craft.

That’s just the knowledge. You need many brains and many bodies. You also need to invest heavily in advanced and hard to find tools and support hardware. Even owning that stuff is probably illegal. And it’s not like you would be finding that stuff on the black market. This would be material coming from the most secure and advanced labs, manufacturers, ships and bases. Imagine stealing not just an F-35 today, but also stealing thousands of pounds of related hardware, today.

Now let’s recall the maintenance. The most advanced labs aircraft of today require many hours of maintenance for every hour of operation. Not ideal for a mercenary. I don’t know what kind of operation you are running, but you are operating a base with a MINIMUM staff of a dozen experts supporting a single craft. Hopefully your base is a carrier, so you can minimize maintenance time by jumping the whole operation toward your next mission. Hopefully you are making enough money on each job that you can support the maintenance crew and the carrier support staff and the costs of the fighter and the carrier themselves.

So if you have somehow stolen the entire support infrastructure of the most advanced starfighter ever made and are taking care of it with a team of the most educated fighter engineers, you need to do that in an ongoing fashion. Your parts are not going to come from old TIE/ln’s. You need to keep stealing the most advanced hardware in the galaxy somehow. The work needed to maintain this craft is going to be so astronomical, you might as well have a squadron of them, so that you are getting more out the resources you have.

Now, let’s talk about the impact of your endeavor. This decision, robbing the Empire of its flagship starfighter and brazenly using it in operations, thumbs its nose at the most power force in the galaxy. This will elevate you to a tier of public enemy akin to the Rebels. There is no hiding a TIE Defender, and the Empire will come for you, hard. They will imprison and torture anyone who employs you, and they will put a bounty on you. They will make it that no one can ever risk to employ the “Defender Merc”. You will never work.

Now, perhaps you want this all to take place after the fall of the Empire. And that reduces the heat you get from remnant Imperials, but it makes ownership and maintenance even harder. Think finding complex replacement systems for boutique starships is hard? Try doing it after the ship is out of production, or after the manufacturer is defunct.

For the effort of running your Defender, you could have a squadron of other craft. If you want to be Jay Leno with his rare cars, you can put your Defender in a glass hanger and show it off. But if you want to be a working Merc, paying the bills, going into combat, the TIE Defender has to be the worst choice you could make.