I am going to go a different route. I am much the same way as you. I did the desk/ paper puching job for three years. Hated it. I like working with my ands and seeing progress. At a desk....
When I speak to high school kids about what they might like to do for a living when they graduate, or go to college for, I always recommend find whatever it is that you like to do the most, then find someone to pay you for doing it! Is it always realistic, no. But where I am going with this is regardless of if you are being paid $1 mil for something you hate, or $100 K for something you love, in the end you will still be happier doing what you love! The money will be enough modivation at the start but after a few years that will no longer be enough incentive for your brain to keep you happy.
My suggestion would be do what you would be happiest doing and if it looks like you can get paid very well there too, that is a bonus. But dude, do something that you would like to do even if you were not being paid for it, and something that you do not dread. That way when Monday comes, you dont think Oh Crap, I dont want to go to work. I hate it there.
Trust me. Been there done that.
EDIT: Oh yes, the original question!
Yes, GTS values are down big time! If you can, keep it for now. Mods do not hold the same value when selling time comes. Ironically stock cars do often get the same or more as modded cars get the impression of being abused. I would say $40K would be the high end of what you can get, but if you do sell, take some time and try for what you can get! The right buyer is always out there. I would suggest keeping it, but if you can't...
Robert