I have been using a DII for several years. I took it down to the nuts and bolts to clean and lube everything. Mine runs the rails on standard ball bearings which were essentially frozen with old grease. Several soakings in kerosene got them working again. Movement up and down keeps things free. There are several “adjustments” in the focus mechanism which require attention – don’t loose any parts.
One 4x5 condenser has a mark I cannot eliminate. Placing it in the higher position solved this. Another “problem” was the light socket. It came with one made of brown plastic which placed the light at the wrong level. I understand that the original had a ceramic socket which I could not find for direct replacement but the hardware store had a brass (plated steel, really) with outside screw threads which matched the head exactly. Even then I had to adjust the head position to obtain good light distribution. Once this was done it stayed solid.
In fact, the whole DII is like a tank – indestructible. $100 is probably a good price provided et c. Do not chinch on your lens(es). Get the best you can afford. There are bargains “out there” but caveat emptor – check out any “deals” that seem to be too good to be true. If help is needed feel free to pm me…..