USA local SIM card

Sounds like you should go the mvno or twilio route. Twilio is beta testing cloud based since assignments that tie into their data and SIP services. If you go the direct carrier route try mvno.

1. My phone is not LTE but 3G GSM/UMTS capable (all bands,
850/900/1700/1900/2100). Will it work? Is 3G coverage good enough in New
York and Orlando for VoIP calls (SIP, Viber, Skype)?

3G coverage is a superset of LTE coverage. (aka: carriers still have
some areas that have 3G but not LTE).

AT&T has 850 and 1900 in 3G. the AWS (1700/2100) is for LTE only.
AT&T has shutdown 2G, but T-Mobile still has it.

In Canada, Rogers still has 2G, but Bell/Telus never had 2G. (they went
from CDMA to 3G circa 2010).

T-Mobile does not have 850. It has AWS (1700/2100) and 1900 in the above
list. Originally, it had 1900 only (2G). When it acquired 1700, it
deployed 3G on it. But because the big US carriers deemed 1700 to be for
LTE once it arrived, very few handset manufacturers provided support for
3G on 1700, especially during the days when handsets coudl only support
3 or 4 frequencies. Many of the hansets custom ordered by T-Mobile and
the 3 small new Canadian carriers replaced 1900 support in 3G with 1700
support. (so when Rogers got the Mobilicity customers, many of them had
handsets that could not support 3G services in 1900 so Rogers had to get
a package to upgrade those customers).

T-Mobile has subsequently refarmed 1700 to support LTE, and split its
1900 to support 2G and 3G. It has since acquired some 700 for LTE
service, but this is no help to you. However, as a 3G-only user on
t-mobile, you are limited to 1900 which has shorter propagation from
antennas. So consider that the T-Mobile coverage maps may be built with
700mhz propagation in mind, so you would not get as much coverage on a
1900 only sertvice. There may still be areas where 3G is on 1700, but
propagation is similar to 1900. (assuming your handset can support 3G on
AWS (1700/2100).

Note that AWS (1700/2100) is not used outside North America, even if
frequemncies such as 2100 are. Carefully check your handset's specs.

2. Is there public or private IP address? IPv6?

I can't answer this. During my bike trip, I choose AT&T because it is
the service which cuases me the least amount of waiting to post a tweet
or check emails. Getting the IP address on an iPhone isn't easy so I
didn't waste any time doing this.