Assuming you are looking for software, not in-car telematics, and you are willing to integrate rather than build it yourself, you could try using ZenDrive. That would give you a lot more than mileage too (eg, driving behavior).

You can find some generic advice online but you should ultimately get an immigration lawyer. Working with a larger company will make the visa situation easier as they have more resources- even Series A or later. It may be possible to sort it out as a founder, but that is likely more difficult (or at least expensive as I think the E-2 requires a significant investment). Some (eg, Skype, Zendesk) have founded the company in their home country and then moved to SF after they raised funding. If you have a special skill set (eg, PhD) there is also the O-1 visa. Note: I am not a lawyer and this does not constitute legal advice.

The tech and operations are HOW they scaled. But the demand and service quality are WHY they scaled. 100 years ago, anyone with a car could drive you across town for a fee. It was a dangerous job and, left unregulated, became difficult to provide safety, ensure supply/demand matching, etc. So cities stepped in and regulated. Taxi commissions became entrenched, powerful and politically mafia-esque. They focused on core areas where they could get the most business with certainty: airports and hotels. You could only drive if you had a city-issued medallion and cities set caps on the number they issued. Those who had medallions fought cities to prevent new medallions from being issued to reduce supply and increase demand. Supply & demand grew increasingly imbalanced. And, as more young professionals entered cities with fewer parking spaces for cars, demand grew rapidly- but taxis did not serve this market as they focused on hotels and airports. Uber entered via black cabs and then P2P. Neither of which required medallions, thus circumventing the taxi supply/demand constraint. Technology enabled them to overcome the trust and safety issues of previous jitney services, increase speed and reliability, and enabled them to scale to a new market: urbanites. Growing parking constraints from increasing urbanization, increasing ubiquity of smartphones/apps, changing (younger, professional) demographics of cities, and deteriorization of both taxi and transit service quality have all supported the growth and demand for their service. Note that their growth has not been universal across all markets. Even though they are in 300 cities, most of their demand comes from a handful of cities sharing certain similar characteristics for certain types of trips at certain times.

The best time to seek funding is when investors are asking for meetings and you don't need the money. Generally speaking, you want to raise money right after you have done something that increases the value of your company and gives people a sense that 'the train is leaving the station'. You want to raise enough to get you to the next milestone that does the same for the next round.

I have a lot of experience working with specialty insurance brokers who can build custom policies if necessary depending on what you are trying to build (eg, new carshare, rideshare services). If it is a traditional car rental program, you'll need to find a broker who provides that type of insurance. You could try asking local rental companies who they work with and ask for a referral. Having a small fleet and working with low income populations (eg, possibly no credit cards) adds some risk and challenge for getting set up. You said "some" of your cars will be leased. What about the others? The first question you need to answer is 'who owns the cars'. If you are trying to do something peer-to-peer, it will add cost and complexity and there are fewer options for who you can work with. I'd suggest doing one or the other and avoid mixing the two. Happy to discuss in more detail on a call.