The Innovate Motorsports type of sensor measures at the tip only so you don't need to install deep into the exhaust system this style runs cooler and last a longer as long as it stays dry. It's is just a bosch o2 sensor.

You only need one side, but you can do both sides and double the cost of equipment required.

The adapter plate is about 1" thick.

It can be a pain to get the exhaust aligned back up. Straight hose out the back with a little flex will work without too much difficulty. If you have silent choice that would complicate things.

You need one gauge per wide band sensor.

I believe Innovate makes a gauge that will support two sensors

FAST fuelairspark.com has a dual O2 digital for $ 450.00 and for $ 150.00 more you can get the rpm adapter that will overlay rpm's to a laptop. But you do not need a laptop to use the digital unit

Would one of these monitors be good for tuning on a carb engine? Checking plugs is kinda a pain and I just wondered if this would be a solution?

absolutely

yes, going to use to fine tune my carb procharger setup..