Cell provider employee here... I'm not gonna recommend our services, so I guess that makes this not a disclaimer? Also, edited as a result of noticing your link to your phone radios...
Your best bet for having coverage in the middle of nowhere in the United States is Verizon Wireless. Unfortunately, they are a CDMA provider, so if you wanted to go with them, you would need to purchase a compatible phone. This may be worth it for the places you're traveling. (On a recent trip to Italy, I chose to buy a cheap phone compatible with local networks. It was $99 and I re-sold it for $75 afterwards.)
As others have mentioned, the two primary GSM providers in the US are AT&T and T-Mobile. They'll likely have good coverage in cities and on major highways, but you'll be unlikely to have coverage out in the middle of nowhere on back roads. The coverage maps on their websites will be helpful.
It appears your phone would get good coverage for talk/text/3g data on AT&T, but AT&T primarily uses the 700mhz band for LTE coverage, which your phone does not appear to support. (https://en.wikipedia.org/wiki/AT%26T_Mobility#Radio_frequency_summary)
It looks like your radios would be slightly more compatible with T-Mobile's frequency bands. (https://en.wikipedia.org/wiki/T-Mobile_US#Radio_frequency_spectrum_chart)
However, I'm not an engineer, and I don't work for either of those companies, so your mileage may vary.
Whatever you do, I'd recommend sticking with one of the big carriers (Verizon, T-Mobile, AT&T, or Sprint) or one of the brands/companies that resell service from the big carriers (Cricket, MetroPCS, etc.). There are some smaller carriers (US Cellular) that only have good coverage in certain areas - avoid these. Wikipedia should be your friend here.
As for your specific questions...
- Yes, you can definitely buy a SIM separately from a phone. Providers will charge you anywhere from $0 to $15 for this.
- Starting credit will not be included with a SIM. Usually, in the US, we just have a concept of a monthly plan. You'll want to look for prepaid plans specifically. You can pay for one month with no credit check, and simply cancel before your next month of service.
- Yes, you can definitely get prepaid plans that include data.
- In the US, it's most common for people to sign up for wireless service at the store of one of the major wireless providers. Any relatively large city is likely to have at least one store from each carrier, and you could probably get up and running in well under an hour. So, yes, you can walk in and walk out with a US phone number in a matter of minutes.
- Something around $30 is probably doable. T-Mobile offers a prepaid plan with unlimited talk & text, plus 3gb of LTE data for $40/month. https://prepaid-phones.t-mobile.com/prepaid-monthly-plans?icid=WMM_PD_SMPLYPRPD1_AAW88WBN93755 So, you could walk into a T-Mobile store, buy a SIM (you might be able to talk them into giving it to you for free), tell them you want to sign up for this plan, and be on your way.
Update about payment... I just did a quick Sales Chat with a T-Mobile representative. You do not need a US billing address in order to sign up for a prepaid plan, but you do need a US credit card. A prepaid VISA or AMEX card will suffice for this. I imagine other US carriers have similar policies, but I'll let OP do the legwork on that. Chat contents below:
You: If I were to travel to the USA, and I had an unlocked,
compatible, GSM phone, would I be able to sign up for a prepaid or
pay-as-you-go T-Mobile plan?
Eldwin: Thank you for visiting T-Mobile
today! I would be happy to provide you with information regarding the
phones you can use with T-Mobile.
Eldwin: Yes, you can.
SIM cards are a great option if you already have a phone and would
like to bring it to T-Mobile. Our only requirement is that your phone
is GSM-capable and Unlocked, so that you can start using T-Mobile
You: Even without a US billing address or credit card?
Eldwin: You will need a valid US credit card to process and order
You: I see.
You: There's no way to pay my bill in store?
Eldwin: You can pay and order in store though.
You: OK. Would it work
to use a prepaid VISA debit card?
Eldwin: I recommend that you order
here online since it is easy and one less trip to make.
it would word.
You: Great - and I wouldn't need a US billing address?
Eldwin: Yes, that is correct.
You: Awesome, thank you!
Eldwin: You are
welcome. Have I answered all of your questions regarding our services?
You: Yes, thank you.