I'm sorry to hear about your divorce.
- I expect I need a job and bank account to get a lease; probably need proof of address to get a bank account/license. Where's the best place to start?
You'll definitely need a job and bank account to get a lease, as well as some money in the bank for application fees and deposits (varies, but typically 1 month's rent + pet deposits if that applies to you). Many landlords / leasing companies require your monthly salary to be at least 3 times your monthly rent but can go up to 5 or 6 times (or more) for some luxury properties.
You will be able to get a bank account using your friend's address. I recommend walking into a bank branch and opening a bank account in person. If you try to do it online you probably won't be approved because they'll have a hard time validating your ID off your credit report alone. You should be able to open a bank account with your passport and social security number. You probably will not need proof of address.
Check your state's DMV website for your licensing requirements. Expect to have to take whole test again if your previous license expired a long time ago. If you still have your old US license, you can take it with you and see if they will exchange it for one in your new state without having to take the exams. It just depends on your new state and how long your license has been expired. You may need some proof of residence for the license, but your bank account should suffice. There may also be a form your friend can sign that verifies you are living at their address.
- Will my UK credit (decent) follow me back to the US? Or will I be starting at zero? If I'm starting over, any recommendations on the best way to start building credit in the US today, although I'm still in the UK for a few months? (And don't have a US address?)
Your UK credit will not follow you back to the US. There are some banks that have Expatriate services departments that will pull UK credit reports to decide if they will open US credit accounts, but these tend to be top tier services for bank members with tens of thousands in deposit at the bank. Still, it's worth asking the regular consumer credit departments if they can help. Companies like American Express, HSBC, RBS, and Barclays all have solid presence in the US and may be willing to transfer your UK account to the US or use your UK credit in consideration of a new US account. People have been successful with this in the past.
As for your US credit, it depends on how long you've been gone and if you kept any credit lines open while you were in the UK. It would be worthwhile to pull a 3-bureau credit report with FICO scores from someone like CreditCheck Total, myFICO, or one of the 3 bureaus: Experian, Equifax, TransUnion. If you see any errors on your reports, fix them. An OK score that needs some help will be in the 600s. A good score is in the 700s or above. With a job and a score in the 700s you'll probably be able to open credit cards pretty easily.
If you're in the 600s or below and finding it hard to get an unsecured credit card after 2 or 3 attempts (be sure to read the qualification guidelines for each card before you apply - some have minimum credit score requirements), you might want try a secured credit card for 6 months to 1 year, where the bank will automatically open up an unsecured card for you after you've proven that you've managed your credit and payments well. You may also be able to get a car loan in the 600s but your rate will be better in the 700s.
You wont be able to open US unsecured credit cards or loans until you have proof of US residence, income, a bank account, and possibly a Driver's License or US state ID. Some banks may require you submit this info in the application or supplementary to the application, especially if your US credit report is looking slim. I don't think there's much you can do right now from the UK, but once you get back to the US it should move along quickly.
Check out myFICO.com for lots of info on how lenders asses your credit, how to build it, and how to maintain it. Run those reports and scores - this is definitely something you can do now b/c correcting errors can take a while.
- If I move back in September, will I get a penalty because I didn't enroll in ACA last month? Or I get some kind of grace period to establish residency and sign up? ACA didn't exist when I moved away. (I don't plan to be working for at least a few months after moving, if that's relevant.)
I don't even know if you can enroll in ACA now. The best thing you can do is check out the exchange website for your new state and see if there's information on special enrollment circumstances. This might fall under a "change in circumstances" (or something to that effect). You're in uncharted territory with a lot of other folks. There's always open market insurance plans and short term policies to hold you over until you're employed. Just be wary of short term plans, they are meant to protect you in catastrophic situations only and may not pay out for regular health care or minor illness.
- I'm not really planning to ever move back to the UK, but as I am eligible to be a citizen, I think it might be wise to get it before I go, just in case. Will I then retain that forever, even if I only visit occasionally? Any other thoughts about this? (It's a lot of money. Maybe it's not worth it at this point?)
Absolutely get your citizenship before you leave. Start now - the process can take a little while. When I did my citizenship something like 13 years ago, I actually qualified the day before I left the UK! It was by the skin of my teeth. I sent my application from the US and did my naturalization ceremony at a US Consulate. That was back before the UK did citizenship tests and interviews though. So be sure to check how long applications are taking to complete and if you can do any interviews, tests, and naturalization ceremonies abroad. If not, you could theoretically go back to the US and return to the UK as needed for these formalities, but there's the cost of the plane tickets and hotels to consider.
I've seen a lot of people say they were never going back and decided not to get their citizenship and then regretted it. Always better to get it and never use it than not have it when you really want or need it. I didn't think I was going back either and well... lets just say I'm glad I have my Citizenship!
Timing the Citizenship is probably the hardest thing on your list. Everything else is just a bunch of paperwork and showing up places with ID! You'll do fine and it'll all settle into place pretty easily. Good luck on your move back.