You can focus with the viewfinder by checking in which places the viewfinder image turns unsharp, then choosing the middle of the two points at which you start seeing its out of focus. This was enough on my D600 and is enough on my D750 to manage sharp pictures of static subjects.
You can get focus confirmation from the AF system of the camera.
You can get a magnifier for the viewfinder from Nikon; the one for the D700 probably works fine, the one for the D750 though seems to be worthless.
You can get focusing screens from secondary companies like KatzEye, though installing them needs a minimum of skill and of course also violates the warranty.
You can focus perfectly with lifeview and magnification. I think the D700 already has lifeview, as one of the first cameras. I dont know how well it does magnification. The issue I'm having on my D750 is that choosing the location to magnify is EXTREMELY SLOW, which is very annoying. Other than that, it works beautifully. I use it in low light.
You can also use devices like the "Zacuto 2.5x Pro cine-finder" (which Ming Thein uses with the D810) to make it easier to see if you're in focus, especially in bright sunlight.
I am currently in the process of trying to get my first pure manual focus lens. Many of the best lenses for Nikon F are actually purely manual (Zeiss, Voigtländer/Cosina, old Nikon AI-S like the famous 105mm f2.5), so if I'm happy with the results, theres indeed a lot of great glas to get.