I've got the Fender Mustang IV v2 and Fender Mustang III v2, which I kept even after I got a Helix Floor several years ago. I do not think that your Fender Mustang I V2 has a line in. Nonetheless, as it is a v2, it may have the Studio Preamp Preset. If it does, that Preset bypasses all the DSP (tone stack and speaker cab emulation) in the Mustang amp. It has been years since I looked at the feature set of the Mustang 1 v2 amp, so if that Studio Preamp Preset is not available, then there may be no way to disable the amp modeling and speaker cab modeling in your Mustang. Thus, if you were to use any Helix Presets containing amp blocks or cab blocks, those would be routed into the front end of your Mustang 1 and the Mustang would then layer it's selected Preset's amp and cab modeling too. The result may not be what you want. On the other hand, if it sounds good then go with it.
Typically, when I run my Helix with either of my Mustang Amps (which I do not often do anymore, as I far more frequently use FRFR PA Speakers or Studio Monitors) I disable the Mustang's DSP and Spkr cab modeling, either, by using the Studio Cab Preset (mentioned above), or, simply by routing Helix's output to the Mustang's Line In or Aux In jacks. In the latter case, the Aux In jack automatically bypasses the Mustang's entire DSP modeling. But, again, I do not recollect if the Mustang I v2 offers an Aux In jack. If it does, simply use a 1/4" TRS from Helix's Headphone Output to a 1/8" TRS to the Mustang's Aux In, or, a 1/4" TS L and 1/4" TS R from Helix's L&R outputs to a 1/8" TRS to Mustang's Aux In. All of these cables are inexpensive and readily available, for instance, via Amazon.
Overall, the Helix gear is phenomenal! I concur that it will blow away any of the modeling the Fender Mustang's offer. However, I kept mine to use as powered cabs and they do serve that purpose, as well as being backups should the Helix gear fail.
Hope this all helps in your considering your options.