What would the rough estimate be of those aftermarket parts for the fastest option? What would the lowest gear start out at?
And how reliable would this be compared to a stock bike?
I am new to biking etc.
Ah, you're new to this. Have you ever ridden a bike with 16” wheels? I have a Dahon Curve D3 converted to a SRAM 2-speed Automatix hub, and a Brompton 3-speed with electric assist. I live on a very steep hill, so it's quite easy to get these bikes going faster than I'm comfortable with. The most I'm comfortable with on those little wheels is about 20mph before things start to feel squirrelly.
In my opinion you should find a folder you think will work for you, and not worry too much about top speed right now. My guess is you'll discover that the stock gearing will match the abilities of the bike.