I have a 2004 Trek Navigator 200 that I'm putting together. It needs a rear derailleur. My Trek frame has a direct mount hanger on it. Do I have to have a direct mount rear derailleur or can I use a standard mount rear derailleur? With or without b-link? Can I visually tell the difference between a standard mount RD from a direct mount RD?
The 2004 Trek Navigator 200 and 300 features an 8 speed derailleur system drivetrain. The 200 specifically uses Shimano Alivio 8 speed. This derailleur is mounted conventionally with the derailleur fixing bolt threading into the bike's derailleur hangar.
The term "direct mount" derailleur is a bit confusing. The 200 and 300 Navigators use a conventional direct mount derailleur. This is the "normal" or most common set up in modern multi speed bikes, and not the more recent iteration of direct mount frames that require the rear derailleur to mount differently.
If you want to remain at 8 speed in the rear, you simply have to have an 8 speed derailleur that matches the brand of shifter you have. The 200 model used a Shimano Alivio system and the 300 used SRAM 8 speed X7. Because of differences in the amount of cable pulled by the shifter and the engineered response of the rear derailleur to the amount of cable pulled, you can't mix Shimano and SRAM when dealing with rear derailleurs and shifters. They must be the same brand. And with SRAM, they have various systems that aren't compatible with each other. If you find you have a SRAM X7 shifter, you'll require an X derailleur compatible with the ESP system of SRAM.
The best would be to buy a standard derailleur hanger to replace the direct-mount one. They are available from a variety of places, including Wheels Manufacturing, BETD, Pilo, etc.