Welcome Dexterdobe. I am with Bhkyte on this. I have basically done what you are describing, albeit converting and I4 to Sram dual drive. My advice is do not get an I4 get a D9 instead.
To convert an I4 into a D9 you need:
- A new wheel or rebuild the old rim into a new hub.
- A 9 speed cassette.
- A rear mech.
- A new rear mech hanger or file/machine the old one down as its too fat.
- A 9 speed shifter.
- A larger chainring because the existing will be under-geared.
- Potentially a longer chain, especially if you go for a larger chainring.
- A lot of spare time and money.

I did all the work myself but I have the skills and equipment to do it but I wouldn't recommend it because of the time involved and because the price difference between the two models is less than the cost of the additional parts.
If you wanted to convert to a hub gear with more gears such as 8 or 11 then an I4 would be as good choice as an D9.
I would like to try converting either model to a triple chainring, I think it could be done quite cheaply.
Excellent choice of fold bike though, they are well built, compact and a fraction of the price of a more well known small wheeled folding bike. The Mezzo also has a much bigger and more useful rear rack for touring. My dual drive mezzo with bull bars and Kojak tyres is quick and efficient.

Good luck.