Wow, I had no idea you could get the Maserati for 50k now. That would be an awesome choice, IMO, but I know nothing about what it takes to maintain one.
After reading many peoples build threads, Ive come to the opinion that the FF GTM is shit and can't be built for 50k. Don't do it. Seems like half the people that set out to build one end up switching to the Superlite SLC and have nothing but great things to say about it. It's extremely fast, but over your price range.