There will be frugal minded comments aplenty upcoming but I'll give you my take.
HLT: thermo and sightglass kinda mandatory. You need to know your temps and levels. Yes, you could use a handheld thermometer and use a story stick for levels.
MLT: Thermo maybe, but I've noticed my dial thermo is a little slow to respond and you're only measuring one point in the grainbed. I always doublecheck with a digital handheld and they're sometimes off by 2F. Sightglass? Only if you plan to fill the MLT and heat the strike water directly. If you're going to heat in the HLT and move it to the MLT, you don't need the sightglass.
Boil Kettle: Thermo...opinions plenty mixed. I like to know when I'm about to boil or how close I am. If you chill in place with an IC, it might be nice. I'd only add one if I found a few of them cheap. Sight glass... again, you could use a stick or notch your spoon etc, but if you're building three vessels anyway, it's really nice to have a quick indicator of how much runoff you have and how much you've boiled down.
Me? I made all three vessels identical. They can be used interchangeably and if I ever sold them off, they would be advertised as universal HLT/BK/MLTs. If you're having couplings welded in, it's not much more expensive to put 9 bulkheads in than say 6.