a sightglass is useful in the HLT if its up high (like in a 3-tier setup). the mash tun doesnt need one as the differing amounts of grain will change the scale, and it will be hard to clean. its usefull in the boil kettle because with the turbulance of boiling, its difficult to tell where the fluid level really is.
you dont need a bottom of any kind in the boil kettle. you can put a screen on the outlet port to prevent sucking up hops and debris, but dont use a whole false bottom.
i will not use MIG on any brewery related connections. some people might half-ass it like that, but its sanitary TIG or nothing for me. some people would argue that its the boil kettle and doesnt matter as it gets sanitized anyway... its up to you. if TIG were out of the question, i would silver solder or braze the connection before using MIG.
I plan using 1/2" weldless fittings on the keg for my ball valves.
you can use the same fittings for the in and out of the HERMS coil. or else run the ends up to the top of the keg, above the water line, and put some sort of connection on the top. i used 1/2" copper for my coil, so i soldered on 1/2" sweat-to-NPT fittings, and then screwed camlocks onto the copper.
remember HERMS pipe legnth is not always longer=better.