To mod the Tern C7 to 18 or 20 speed, you have to change the wheelsets as well. You can change the hubs alone if you want to save but the rims are also not a good one. Usually they pair the rims with single walled rims which are soft and broad as well. You can't fit the Duranos or any narrow tires on it. So, it makes better sense to just change the whole wheelset. A sealed bearing set can be had for RM400 , give and take 50 bucks. As for 18 speed or 20 speed, that depends on you.
For 18 speed, it means 11T/32T or 34T rear cassette (cogs) and you can use mountain bike RDs. This means shifters, chains and RDs are aplenty to choose from. The front most definitely be road bike set up. A 52T or 53T/39T chainrings. You can't find mountain bike FDs that can fit the big diameter seat tube anyway. So, roadbike FDs are used. It will be much cheaper, but not much.
For a 20 speed, means you use roadbike components all over. 10 speed mountain bike components are already 34T or 36T for the rear cogs which are a bit big as the cage of the RD will hit the ground. So, roadbike components are more suited for 10 speed. The biggest is 30T at the rear which should suffice for a 20 inch foldie. It will cost a little bit more, namely the Shimano Tiagra is the cheapest 10 speed outfit, then comes the Shimano 105s and then Ultegras and off course the top Dura Aces. All the latest models of the top 3 models are now 11 speed already and that requires different hubs,

Unless you can find older versions, I guess you have no choice.
I did manage to fit in 10 speed SRAM X5 mountain bike components on the 20 inch foldie though as SRAM X5 RD has a shorter cage. Also you need to shorten the length of the chain in such a way that when you engage the lowest 36T, the cage is pulled to the front and not vertical to the ground. This makes going uphills .........hyper easy.

But shifting is not as crisp as the Shimanos. Other than that, it's fun. Roadbike components are designed to shift very smoothly. They are precision gears. I ride mountain bikes with Shimano XTs and they are not even comparable to Shimano 105s when gear shifting. Maybe it is also due to the small ratio changes.
It's going to cost as much as what you paid for the Tern but buying a ready made Tern with similar specs are even much more pricier.

So, be prepared for that.

sifu etigge, in order to use 11t/32t cassette, is tat mean we hv to change the rd to long cage? tq