In agreement with both of Andy's comments, if you have not ridden a bike with 20" wheels before, believe me it is a very different experience from driving 26" or 28" wheels. Specifically you wobble/wander all over the road. If I take a glance back over my right shoulder to see where my wife is, when I look forward again I usually find I am heading for the opposite kerb. (Answer: buy a handlebar mirror!) A similar thing happens when you stick out your arm to indicate a left or right turn! For that reason we usually ride down a few small/low traffic lanes near home and very much keep off main roads. Luckily we also have quite a few tarmac surfaced shared cycle/walking pathways nearby. Google Sustrans for good ideas.
We have Raleigh Stow-E-way folding bikes. If you are tall - I am 6'1" - the standard saddle shaft only accommodates riders up to about 5' 10". The saddle shaft should be marked with min and max extensions: if you cannot accommodate the max setting with your leg length, longer marked saddle shafts are usually available but DO NOT ride with the saddle shaft extended beyond the highest level or you could damage the saddle shaft or its mount on the bike (not covered by warranty!) Also male saddles these days tend to be akin to razors, so get a wider saddle (usually a woman's) if you want more comfort (often on offer at Aldi or Lidl.)
If the bike you buy has bottom bracket (pedal) drive - such as the excellent Bosch system - you have the advantage that you can have a double clanger on the bottom bracket. (Oh, come on? A Double Clanger is two cogs on the pedals which doubles the number of effective gears you can achieve with the usual Derailleur gears on the back wheel.) However if you have such a system the pedal drive system requires leg pressure on the pedals before it provides assistance. On the other hand if you have rear wheel drive which has only eight gears (give or take,) once you have the pedals turning you usually don't actually have to apply pedal pressure - just keep the pedals moving and the motor will keep the bike moving. (

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Finally for safety not only should you wear head protection when riding, you also need eye protection, learn to ride with your mouth closed unless you like the taste of flies various, gloves to keep you hands warm (I joke not!) and if you are prone to not wearing bright/light clothing a hi-viz vest or tabard makes you more visible (it is amazing how many drivers just do not see you!) A good bonus is a speedo. However most such devices have wire connection between the display unit and the wheel sensor which is a problem (again if you are tall) if you need to raise the height of the handlebar. Better to get a kit that uses Bluetooth to connect the data flow.
Not advertising, but all of the items mentioned in the above paragraph are easily available from a branch of Decathlon. A simple set that will show your distance travelled, current speed, average/max speed, and cumulative distance can be got for prices in the £15-£30 range and is all you really need.