I had a guy build mine, HLT, MLT & BK, and he talked me out of a sight glass on any of them. I'm glad he did. The kegs are kind of cumbersome and I would have surely broken at least one by now. Someone with a better setup, maybe the ability to CIP, might say different, but I get along fine with my mash paddle/dipstick to keep track of volumes. I think a sight glass in the mash vessel would be more trouble than it's worth to keep clean, though.
The same guy showed me how he recircs his RIMS by laying the silicone tubing on top of his grain bed, over the top of the keggle rather than through a fitting. If I ever recirc I'll start out the same way. I can always take a keg back to have another fitting or two installed if I think I need it. His rates are pretty reasonable.
One thing I think I wish I'd have done on the MLT was to cut out the bottom and drain through the original top opening via a triclover fitting. It's not that bad disassembling the dip tube and pulling the false bottom for cleaning, but I might do things differently if I were to do it over.
