Hardest I found is where you want to place them in the cabin. A-pillar or triple cluster etc. I think Apillars gauges are illegal though.
Work this out first. Also what size, colours, brands and stuff. I'm a stickler for consistancy.
Boost is easy. Mechanical is better is my opinion. Had an electronic one stuff up and now tainted.
I wouldn't worry about air/temp as just pretty lights unless its a wide band.
Always have a Oil temp. Helped my out once when the thermo stopped and the oil temp worked faster then the factory water temp.
I do my own but have friends go to leMans in Brisbane.