It’s certainly easiest to have the metering fitted when the heat pump is installed, but that’s not to say it can’t be fitted later. Somewhat trickier if glycol is added to the system.
The benefits of O.E.M. bundle is that it gives you accurate, high resolution data, plus out-of-the-box integration into emoncms. Great it you want to be sure the heat pump is working at its best, and so that you can make tweaks to the system to improve it. Anything else is a compromise on ease or data quality.
For example, here are snaphots from three different Ecodan systems that use different meterings: (click to embiggen)
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level 3 metering from OEM - data recorded every 10 secs
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Mitsubishi metering via Melcloud - data recorded every 2 mins
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estimated power via Melcloud - data recorded every 2 mins
It largely depends on what you want out of it. Melcloud data will provide just enough information to know if the heat pump is working as it should, and the estimations of energy will help guide tweaks and adjustments, but not necessarily be comparable to other installations.
It also depends on how much of the integration you want to do yourself, your technical ability and desire to combine with other systems in your house.
The most common heat pump issue I’ve seen discussed is it being oversized for the property, which is something you can only really address before it’s fitted. Pay close attention to the heat loss calculations, especially assumptions about air change rate (ACH), and ask your installer if anything doesn’t seem right.