Recommendations on pyranometer In-amp design
A voltage divider will work fine. For example, you can use a 100 ohm pot paralleled with 10 ohms metal film and 150 ohms metal film in series with battery or other 1.5V supply. It will drift a bit if you use a single battery directly, but you could use two batteries in series and a regulator such as an MCP1702.

It's not clear to me that you actually need an instrumentation amplifier. A simple non-inverting amplifier may be better, but it depends on your circuit arrangement and if there is any wiring to the sensor, and so on. A low pass filter is probably a good idea in any case.

A DMM with a 200mV scale is a good way to measure the input signal.